List of Illustrations

1.1 An idealized exposure-response relationship.

3.1 A simple conceptual model of exposure to hazardous substances: (a) inhalation; (b) dermal; (c) ingestion; (d) injection.

4.1 The ratio of near- (NF) to far-field (FF) exposure levels in different conditions.

4.2 Exposure over a working shift.

7.1 Results from ten measurements of inhalable dust in a packing plant.

7.2 Results from 100 measurements of inhalable dust in a packing plant.

7.3 A log-probability plot showing data from ten measurements of exposure during bag filling.

9.1 The ISO/CEN/ACGIH sampling conventions for health-related aerosol fractions. (Reproduced with permission from Occupational Hygiene, 3rd edition, edited by Kerry Gardiner and J. Malcolm Harrington, Blackwell Publishing Ltd., 2005, p. 187.)

9.2 Dust sampling equipment worn by operator, with IOM inhalable sampler in blown-up inset. (Reproduced with permission from SKC Ltd.)

9.3 (a) Multi-orifice sampler (HSE). (b) IOM inhalable sampler (HSE). (c) Conical inhalable sampler (HSE). (Reproduced with permission Monitoring for Health Hazards at Work, 3rd edition, by Indira Ashton and Frank S. Gill, Blackwell Publishers Ltd., 2000. pp. 6–7.)

9.4 Cyclone respirable sampler (HSE). (Reproduced with permission Monitoring for Health Hazards at Work, 3rd edition, by Indira Ashton and Frank S. Gill, Blackwell Publishers Ltd., 2000, p. 9.)

9.5 The CIP10-R respirable dust sampler. (Reproduced with permission from Occupational Hygiene, 3rd edition, edited by Kerry Gardiner and J. Malcolm Harrington, Blackwell Publishing Ltd., 2005, p. 194.)

9.6 The MRE 113 respirable dust sampler (Reproduced with permission Monitoring for Health Hazards at Work, 3rd edition, by Indira Ashton and Frank S. Gill, Blackwell Publishers Ltd., 2000, p. 9.)

9.7 The cowl sampler used for asbestos and other fibrous dusts. (Reproduced with permission from Occupational Hygiene, 3rd edition, edited by Kerry Gardiner and J. Malcolm Harrington, Blackwell Publishing Ltd., 2005, p. 203.)

9.8 (a) A TSI Sidepak direct-reading dust monitor (Reproduced with permission from Occupational Hygiene, 3rd edition, edited by Kerry Gardiner and J. Malcolm Harrington, Blackwell Publishing Ltd., 2005, p. 197.) (b) SKC Split 2 sampler (Reproduced with permission from Occupational Hygiene, 3rd edition, edited by Kerry Gardiner and J. Malcolm Harrington, Blackwell Publishing Ltd., 2005, p. 197.) (c) Casella Microdust sampler. (Reproduced with permission from Casella Measurement Ltd.) (d) Output from a Sidepak sampler showing variation in aerosol concentration during work task.

9.9 An electronic flow calibrator. (Reproduced with permission from Casella Measurement Ltd.)

9.10 Apparatus used to calibrate a rotameter. (Reproduced with permission Monitoring for Health Hazards at Work, 3rd edition, by Indira Ashton and Frank S. Gill, Blackwell Publishers Ltd., 2000, p. 16.)

9.11 Typical rotameter calibration chart.

9.12 The Walton-Beckett eyepiece graticule. (Reproduced with permission Monitoring for Health Hazards at Work, 3rd edition, by Indira Ashton and Frank S. Gill, Blackwell Publishers Ltd., 2000, p. 28.)

9.13 Acetone vapouriser used to prepare samples for microscopic analysis. (Reproduced with permission from JS Holdings.)

9.14 Layout of Tyndall beam apparatus (A&G Marketing). (Reproduced with permission Monitoring for Health Hazards at Work, 3rd edition, by Indira Ashton and Frank S. Gill, Blackwell Publishers Ltd., 2000, p. 33.)

10.1 A single-gas detector with optional datalogger. (Reproduced with permission from Draeger Safety UK Ltd.)

10.2 Typical impinger samplers (SKC Ltd). (Reproduced with permission Monitoring for Health Hazards at Work, 3rd edition, by Indira Ashton and Frank S. Gill, Blackwell Publishers Ltd., 2000, p. 54.)

10.3 Adsorbent tubes. (Reproduced with permission from SKC Ltd.)

10.4 Badge type diffusive sampler. (Reproduced with permission from 3M United Kingdom PLC.)

10.5 Tube-type diffusive sampler. (Reproduced with permission from Draeger Safety UK Ltd.)

10.6 Colorimetric detector tube sampler and a range of tubes. (Reproduced with permission from Draeger Safety UK Ltd.)

10.7 Gas-tight sampling bags. (Reproduced with permission from SKC Ltd.)

10.8 A portable Fourier-transform infrared analyser. (Reproduced with permission from Quantitech Ltd.)

10.9 A MultiRAE photo-ionisation monitor. (Reproduced with permission from RAE Systems UK)

11.1 A multi-stage liquid impinger. (Reproduced with permission from Burkard Manufacturing Co. Limited)

11.2 Cascade impactor for bioaerosols. (Reproduced with permission from Casella Measurement Ltd.)

12.1 Typical arrangement of patches used in patch-sampling methods. (Reproduced with permission Occupational Hygiene, 3rd edition, edited by Kerry Gardiner and J. Malcolm Harrington, Blackwell Publishers Ltd., 2005, p. 395.)

13.1 Change in acoustic pressure with time for a pure tone. (Reproduced with permission from Occupational Hygiene, 3rd edition, edited by Kerry Gardiner and J. Malcolm Harrington, Blackwell Publishing Ltd., 2005, Figure 17.1).

13.2 Change in acoustic pressure with distance for a pure tone. (Reproduced with permission from Occupational Hygiene, 3rd edition, edited by Kerry Gardiner and J. Malcolm Harrington, Blackwell Publishing Ltd., 2005, Figure 17.2).

13.3 The weighting curves used in noise measurement. (Reproduced with permission from Occupational Hygiene, 3rd edition, edited by Kerry Gardiner and J. Malcolm Harrington, Blackwell Publishing Ltd., 2005, Figure 17.8).

13.4 Simple sound level meters. (Reproduced with permission from Casella Measurement Ltd.)

13.5 Octave band monitoring with a sound level meter. (Reproduced with permission from Casella Measurement Ltd.)

13.6 A noise dosemeter. (Reproduced with permission from Casella Measurement Ltd.)

13.7 A selection of hearing protection devices (muffs, reusable inserts and disposable inserts). (Reproduced with permission from Moldex-Metric AG & Co. KG)

14.1 x, y, z coordinates. (Reproduced with permission from Occupational Hygiene, 3rd edition, edited by Kerry Gardiner and J. Malcolm Harrington, Blackwell Publishing Ltd., 2005, p. 252.)

14.2 The relationship between displacement, velocity and acceleration for a simple mass and spring system.

14.3 Vibration measurements in practice. (Reproduced with permission from Bruel & Kjaer UK Ltd.)

15.1 Kata thermometer. (Reproduced with permission from Ashton and Gill)

15.2 Globe thermometer with glass thermometer. (Reproduced with permission from Ashton and Gill)

15.3 Integrating heat stress monitor. (Reproduced with permission from Quest Technologies)

15.4 Portable heat strain monitor. (a) monitor on belt (b) schematic arrangement of sensors (Reproduced with permission from Quest Technologies)

15.5 Arrangement of thermometers on a stand. (Reproduced with permission from Ashton and Gill)

16.1 The light meter. (Reproduced with permission from Castle Group)

17.1 The electromagnetic spectrum.

17.2 Decay of an unstable nuclide to a stable one. (Reproduced with permission Monitoring for Health Hazards at Work, 3rd edition, by Indira Ashton and Frank S. Gill, Blackwell Publishers Ltd., 2000. pp. 165.)

17.3 Decay of carbon 14. (Reproduced with permission Monitoring for Health Hazards at Work, 3rd edition, by Indira Ashton and Frank S. Gill, Blackwell Publishers Ltd., 2000. pp. 165.)

17.4 An example of decay in stages: lead to bismuth to polonium to lead. (Reproduced with permission Monitoring for Health Hazards at Work, 3rd edition, by Indira Ashton and Frank S. Gill, Blackwell Publishers Ltd., 2000. pp. 166.)

17.5 A Geiger-Muller counter. (Reproduced with permission from Thermo Fisher Scientific.)

17.6 A scintillation detector. (Reproduced with permission from Berthold Technologies (UK) Ltd.)

17.7 Two thermoluminescent detectors (TLDs). (Reproduced with permission from Mirion Technologies, Inc.)

17.8 Two film badges. (Reproduced with permission from Loxford Equipment Company Ltd.)

18.1 A broad-spectrum UV monitor along with the weighting. (Reproduced with permission from LOT-Oriel Ltd (for International Light))

18.2 A hand-held monitor for IRA and part of IRB. (Reproduced with permission from LOT-Oriel Ltd (for International Light))

18.3 A leak monitor for microwave ovens. (Reproduced with permission from ETS-Lindgren)

19.1 Diagram showing the incorrect placement of a hood while welding, i.e. too far from the source.

19.2 Operators placing their head between the emission source and the extraction hood.

20.1 Portable inclined manometer.

20.2 Magnehelic diaphragm pressure gauge. (Reproduced with permission Monitoring for Health Hazards at Work, 3rd edition, by Indira Ashton and Frank S. Gill, Blackwell Publishers Ltd., 2000. p. 89.)

20.3 Digital micromanometer. (Reproduced with permission from TSI Instruments Ltd.)

20.4 Electronic vane anemometer.

20.5 Heated sensor anemometer. (Reproduced with permission from TSI Instruments Ltd.)

20.6 Pitot-static tube. (Reproduced with permission from JS Holdings.)

20.7 Principle of operation of pitot-static tube. (Reproduced with permission from Occupational Hygiene, 3rd edition, edited by Kerry Gardiner and J. Malcolm Harrington, Blackwell Publishing Ltd., 2005, p. 443.)

20.8 Measuring positions for placing pitot-static tubes in circular ducting. (Reproduced with permission from Gardiner and Harrington.)

20.9 Log-Tchebycheff rule for traverse points in a rectangular duct. (Reproduced with permission from Gardiner and Harrington.)

20.10 Face of booth showing measurement positions. (Reproduced with permission Monitoring for Health Hazards at Work, 3rd edition, by Indira Ashton and Frank S. Gill, Blackwell Publishers Ltd., 2000. p. 106.)

20.11 Extract slot showing measured air speed results and plotted contours. (Reproduced with permission Monitoring for Health Hazards at Work, 3rd edition, by Indira Ashton and Frank S. Gill, Blackwell Publishers Ltd., 2000. p. 110.)

21.1 A selection of respiratory protection: (a) disposable respirator FFP1; (b) half-mask respirator; (c) powered hood TH2; (d) full facepiece respirator; (e) full masks breathing apparatus. (Reproduced with permission from Draeger Safety UK Ltd.)

21.2 Face-fit testing kit - PortaCount. (Reproduced with permission from TSI Instruments Ltd.)

List of Instruction Sheets

Chapter 9

9.8 Calibration of a rotameter or electronic flow calibrator by using the soap-bubble method, 76

9.9 The measurement of inhalable airborne dust, 80

9.10 The measurement of airborne respirable dust by using a cyclone sampler, 83

9.11 The sampling and counting of airborne asbestos fibres, 84

9.13 To trace the behaviour of a dust cloud by using a Tyndall beam, 89

Chapter 10

10.5 To measure personal exposure to solvent vapours using an adsorbent tube, 102

10.6 Sampling for gases by using a bubbler, 104

10.7 To measure the short-term airborne concentration of a gas by using a colorimetric detector tube, 106

10.8 To measure a vapour concentration using a diffusive sampler, 108

Chapter 13

13.10 To measure workplace noise using a SLM, 153

13.11 To measure workplace noise using a PND, 155

13.12 To measure the spectrum of a continuous noise by octave band analysis, 157

Chapter 14

14.6 To measure hand-arm vibration, 167

Chapter 15

15.5 Measurement of the thermal environment, 182

15.9 To calculate the wind chill factor, 189

Chapter 16

16.5 To measure lighting, 194

Chapter 20

20.5 Measurement of air flow in ducts, 246

20.6 Measurement of pressure in ventilation systems, 252

20.7 To measure the face velocity on a booth or hood, 254

20.8 To measure the face velocity on a fume cupboard, 255

20.9 To measure the performance of a suction inlet, 257

Chapter 21

21.3 Face-fit testing using a particle counter, 269


The methods for measuring exposure to hazards in the workplace have progressively developed over the last 50 or more years. A major impetus for improving ways of assessing workers’ exposure to chemical, biological and physical hazards in the UK was the introduction of the Health and Safety at Work (etc.) Act in 1974, which required employers to ensure that tasks and work environments were ‘safe and without risks to health…’. Subsequent regulations made under the Act have strengthened the role for measurement as part of a modern approach to health and safety at work. Recent developments have seen greater standardisation of approaches to measurement and an increasing understanding of the role of exposure measurement in the context of assessing risks to health. Over the last 10 years there has been increasing interest in how best to quantify dermal exposure to chemicals and in the future we expect to see more interest in measurement of chemicals inadvertently ingested from contaminated hands or objects. Today, methods for measuring hazards at work are being applied throughout the world, and in the future the process of globalisation and standardisation will ensure greater uniformity in approaches to measurement.

The earlier editions of this book provided a very practical introduction to the topic, with helpful practical advice about how to undertake specific measurements of everything from hazardous airborne dusts to hot stressful environments. We had used the book in teaching occupational hygiene students and to help support other health and safety professionals develop their skills and had found it invaluable. When we were asked to take on the job of revising and updating the text for the fourth edition, we took the opportunity to think carefully about the changes in practice that had taken place and to reflect these in the text while retaining the practical ‘how to do it’ approach that had made the book unique.

The text of this fourth edition has been completely reorganised into 6 separate sections and 25 chapters. The sections cover background material necessary to understand the context within which measurements are made, and then we deal with measurement of exposure to inhalation hazards, dermal and ingestion hazards and physical agents such as noise, heat and radiation. The final sections deal with measurements to assess the effectiveness of risk management measures such as local ventilation or personal protection, and risk assessment/risk communication. We believe that these provide a comprehensive introduction to the topic that will help the reader understand how measurements should be made and their results communicated to the workforce.

We hope that the book will be a useful source for students interested in learning about the practical aspects of occupational hygiene measurements. However, we have also included information on many new techniques and emerging areas of occupational hygiene practice and hope that there is also much that will be useful to established professionals who perhaps relied on previous editions of the book in the earlier part of their career. As with the earlier editions we have tried to ensure that the text is accessible to a wide range of health and safety professionals, including safety advisors, occupational health nurses, ergonomists, occupational physicians and occupational hygienists.

It is important to realise that the measurement methods we describe in this book are tools that can be used to assess the degree of risk and guide further preventative actions where necessary. In doing this the readers will need to use a wider knowledge base encompassing the underpinning legislation, toxicology, other sciences, engineering and management techniques. These topics are beyond the scope of the present work. In addition, there is a need to apply sound professional judgment in deciding what type of measurements will be helpful and in interpreting the results that are obtained. There is no substitute for good academic or professional education or training. In the UK, the British Occupational Hygiene Society () regulates a system of qualifications that covers most of the techniques described in this book. Internationally, the Occupational Hygiene Training Association (OHTA) provides a similar role. There are also relevant postgraduate courses available in universities in the UK and throughout the world.

We are interested to receive feedback from readers about the book and to continue to support those interested in measurement of health hazards and other aspects of occupational hygiene. Visit for more information.

John W. Cherrie

Robin M. Howie

Sean Semple

January 2010


We owe a great deal to the authors of the earlier edition of this book – Frank Gill and Indira Ashton. They recognised the need for a simple practical introduction to monitoring for health hazards at work, and their text provided a wealth of material that we have reused and recycled in this edition. We are also grateful to the many colleagues and friends who have offered advice and provided comments on earlier drafts of the text. In particular, we wish to thank Finlay Dick, Alastair Robertson, Adrian Watson, Adrian Hirst, Richard Graveling, Andy Gillies, Bob Rajan and Brian Crook. Adrian Watson also provided initial draft material for the chapters on lighting and vibration, which was invaluable in revising these sections of the book.

The authors would also like to thank the many companies who have provided information and photographs of measurement equipment to illustrate the book.

Of course, the book would have never been finished without the support of our wives and families, for which we are most grateful.

Units and Abbreviations





Useful abbreviations

ACoP Approved Code of Practice – from the HSE
ACTS The UK Advisory Committee on Toxic Substances ()
ALARP As low a reasonably practicable
ANSI American National Standards Institute ()
BOHS British Occupational Hygiene Society ()
CAD Chemical Agents Directive (details at )
CAS Number A unique identifying number assigned to a hazardous substance by the Chemical Abstracts Service ()
CIBSE Chartered Institute of Building Services Engineers ()
COMAH Control of Major Accident Hazards Regulations ()
ECHA European Chemicals Agency ()
EU European Union (
HPA Health Protection Agency ()
IARC International Agency for Research on Cancer ()
IDLH Immediately Dangerous to Life or Health ()
ILO International Labour Organization ()
IOELV Indicative Occupational Exposure Limit Values from the EU ()
IUPAC The International Union of Pure and Applied Chemistry ()
NOEL No observed effect level
SI Système international d’unités ()
STEL Short-term Exposure Limits (See EH40)
TLV Threshold Limit Value is a type of OEL used in the USA and elsewhere. TLV’s are publisan Conference of Governmental Industrial Hygienists ()
WASP Workplace Analysis Scheme for Proficiency. A quality assurance scheme for occupational hygiene analysis managed for HSE by the Health and Safety Laboratory ()
WATCH The Working Group on the Assessment of Toxic Chemicals, which is a subcommittee of ACTS ()

Abbreviations in the text

ACDP Advisory Committee on Dangerous Pathogens ()
ACGIH American Conference of Governmental Industrial Hygienists ()
ACH Air Changes per Hour (ventilation)
ALARA As Low As Reasonably Achievable
ALI Annual Limits of Intake (radiation)
APF Assigned Protection Factor (respirator)
ART Advance REACH Tool ()
BMGV Biological Monitoring Guidance Values (details in HSE publication EH40, )
BS EN British Standard – European Norm ()
CA Control of Asbestos regulations ()
CE Conformity marking system for products ()
CEN European Committee for Standardisation ()
CFU Colony Forming Units
CHIP Chemical (Hazards Information and Packaging for Supply) Regulations ()
CLW Control of Lead at Work regulations
CNW Control of Noise at Work regulations ()
COSHH Control of Substances Hazardous to Health Regulations (
COSHH-Essentials An initiative from the HSE designed to help small and medium size companies manage supplied chemicals at work (
CVW Control of Vibration at Work regulations ()
DAC Derived Air Concentrations (radiation)
DL Derived Limits (radiation)
DNA Deoxyribonucleic acid
EH40 The guidance document published annually by HSE Books, which contains the complete list of UK OEL’s. EH40 ()
ELF Extremely Low Frequency (radiation)
EMF Electro-Magnetic Field
FFP Filtering Facepiece (respirator)
FTIR Fourier Transform Infrared spectroscopy (chemical analysis)
GHS Globally Harmonised System for the classification and labelling of hazardous substances ()
GMP Good Manufacturing Practice
HPLC High-performance liquid chromatography (chemical analysis)
HSE Health and Safety Executive ()
HSG Health and Safety Guidance publications (from HSE)
HVL Half-Value Layer (radiation)
ICNIRP International Commission on Non-Ionizing Radiation Protection ()
ICRP International Commission on Radiological Protection ()
IOM The Institute of Occupational Medicine. An independent research and consulting organisation in Scotland ()
IR Infrared (also IRA, IRB and IRC to signify parts of the infrared spectrum)
IREQ Required clothing insulation (thermal)
ISLM Integrating Sound Level Meter
ISO International Standards Organisation ()
LAA aboratory Animal Allergy
LEL ower Explosive Limit
LEP,d Daily noise exposure level
LEP,w Weekly noise exposure level
LEV ocal Exhaust Ventilation
LPS ipopolysaccharide
MDHS Methods for the Determination of Hazardous Substances ()
MMMF Man-Made Mineral Fibres. Also known as synthetic mineral fibres or man-made vitreous fibres (those MMMF with a glassy structure)
MMVF Man-Made Vitreous Fibres (see MMMF)
MRE Mining Research Establishment
MSDS Material Safety Data Sheet (see also SDS, which is the preferred terminology)
MDHS Methods of Determination of Hazardous Substances ()
NIOSH National Institute for Occupational Safety and Health ()
OECD Organisation for Economic Co-operation and Development ()
OEL Occupational Exposure Limit ()
OVA Organic Vapour Analyser
PAH Polycyclic Aromatic Hydrocarbons
PF Protection Factor (respirator)
PND Persona Noise Dosimeter
PNIHL Personal Noise Induced Hearing Loss
PPE Personal Protective Equipment ()
PPE Personal Protective Equipment
Ppeak Peak sound pressure level
PTFE Polytetrafluoroethylene
PUF Polyurethane Foam
PVC Polyvinylchloride
Q Quality factor (radiation)
REACH Registration, Evaluation, Authorisation and restriction of Chemicals regulations ()
RNA Ribonucleic Acid
RPE Respiratory Protective Equipment ()
SCOEL Scientific Committee on Occupational Exposure Limits. An independent committee set up by the EU ()
SDS Safety Data Sheet
SEG Similarly Exposed Group of workers
SIMPEDS Safety in Mines Personal Dust Sampler
SPL Sound Pressure Level
TLD Thermoluminescent Dosimeters
TWA Time-Weighted Average concentration (See EH40)
UKAS UK Accreditation Service ()
UV Ultraviolet (also UVA, UVB and UVC to signify different parts of the UV spectrum)
VBNC Viable But Non Culturable
VOC Volatile Organic Compound
WBGT Wet-Bulb Globe Temperature
WEL Workplace Exposure Limits ()
WHO World Health Organization ()

Multiples of SI units

Name Symbol Factor
tera T 1012
giga G 109
mega M 106
kilo k 103
hecto h 102
deca da 101
deci d 101
centi c 102
milli m 103
micro μ 106
nano n 109
pico P 1012
femto f 1015
atto a 1018

In this book SI units are used throughout; however, conversions from Imperial to SI are given in the list of common units. Also, we use the form mg m3 when presenting units rather than mg/m3, although both are acceptable in practice.


1 Introduction


Occupational Hygiene and Risk Assessment

1.1 Introduction

“When you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meager and unsatisfactory kind…”

Lord Kelvin, Popular Lectures and Addresses

Scientists have always known that measurement is fundamental to making accurate statements about the world around us. The pioneers of occupational health were also enthusiastic about measuring exposure, even when this involved considerable effort to get reliable data. In recent years, it has become less fashionable to rely on measurement data and we have seen the development of sophisticated computer-based systems to estimate exposures, or health and safety professionals rely on their judgment to come to a conclusion about the risks in a particular situation. We support these approaches but we also recognise that which was clear to Lord Kelvin more than 100 years ago: measurements can provide a precise, reliable and objective description of a situation that is generally superior to the alternatives.

The science of human exposure encompasses assessment and control of exposure to hazardous agents that arise from work, in the home or elsewhere in the environment. It does not really matter whether you want to measure the exposure to diesel engine exhaust particulate of someone working as a truck driver or the exposure of someone else in the street where the truck is unloading: the underlying science is the same. Where differences do arise, they are in relation to who has responsibility to manage the exposures and what legislative regime applies. Occupational health professionals are concerned with establishing and maintaining a safe and healthy working environment. Occupational hygienists are the occupational health professionals who are focused on the prevention of ill health by intervening in the workplace to eliminate or reduce exposures to hazardous agents. There are other occupational health and safety specialists who may deal with different aspects of health and work, for example occupational physicians and nurses, safety advisors and ergonomists. However, no matter what the specific expertise of the individuals they should all be aware of the principles of occupational hygiene to help them in their job.

Hazardous agents may be chemicals, loud noise, unseen radiations or many other things. The discipline of occupational hygiene groups hazardous agents into three categories: physical, chemical or biological agents. Psychological stressors are generally seen as being outside the remit of occupational hygiene. Physical agents include noise, vibration, electromagnetic radiation, ionizing and non-ionizing radiation, excessively hot or cold environments and abnormal atmospheric pressures. Chemical agents include harmful dusts, liquids, gases and vapours. Biological agents include bacteria, viruses and other materials of biological origin that are harmful to health. For convenience chemical and biological agents are often grouped together as substances hazardous to health.

The basis for occupational hygiene is the link between exposure to a hazard and the risk of illness arising from that exposure, where the ‘hazard’ is the potential for harm and the ‘risk’ is the chance that that harm may arise in a particular situation. In general, it is assumed that the higher the exposure someone experiences, the greater the risk to their health. shows an idealized exposure-response relationship for a hazardous agent, which epitomizes the causal link between these two measures. The point at which the line cuts the horizontal axis is the threshold for this particular agent and exposure less than this value will not cause any adverse effects. It is clear that limiting the exposure below the threshold will prevent anyone becoming ill and in these circumstances an occupational exposure limit (OEL) should ideally be defined at this point; i.e. it is a health-based OEL. Note that in this book we use the term OEL to refer in a general way to limit values for exposure.

An idealized exposure-response relationship.


In practice, the setting of occupational exposure limits is more complex than the identification of a simple unambiguous threshold, but the general principle of restricting exposure below some value, to ensure risk is minimised, is still valid. In some cases we cannot identify whether the exposure-response relationship contains a threshold below which the hazardous agent has no effect; i.e. the exposure-response line passes through the origin on the graph. The main types of hazardous agents that may be in this category are carcinogens that act on the structure of DNA -so-called genotoxic carcinogens. Also, people express a range of different susceptibilities to hazardous agents and although in theory, there may be a threshold for a given agent it may be that some individuals will still be affected at lower exposures. For example, someone with asthma may react to quite low concentrations of an irritant gas when others do not experience any ill effects. Finally, there may be factors that make it impracticable to set a limit at a threshold, for example it may not be technically or economically feasible to restrict exposure to the level of a low threshold and society deems that the benefits from production or use of this material outweigh the health risks from the exposure.

There are many systems that are in place to derive OELs: some are based on national legislation and are used to enforce the law in the workplace; some are national or international advisory limits without any direct link to legislation and some are international limits with the intention of harmonizing practice. In Great Britain, there is a system for workplace exposure limits (WELs) for substances hazardous to health, which are linked to the legal provisions of the Control of Substances Hazardous to Health (COSHH) regulations. Each country in Europe has its own limits, but the European Union has set uniform minimum standards for OELs for some hazardous substances through the efforts of the Scientific Committee for Occupational Exposure Limits (SCOEL), which aims to identify health-based OELs. The European Commission then uses the scientific advice from SCOEL to make proposals for indicative occupational exposure limits based only on the scientific advice given or binding limits where socioeconomic or technical feasibility factors are taken into account in the decision. International bodies provide advisory limits for radiation and other physical agents, and these may then be incorporated into national legislation or guidance.

Harm may be caused by exposure to a hazardous agent. The degree of harm depends on the hazardous properties of the agent, the intensity and duration of exposure and the person’s response to the exposure. Exposure is conventionally characterised by two independent factors: the intensity of the exposure and the duration of the individual’s exposure. For chemicals, where the person inhales the substance, the intensity is usually the average concentration of that substance in the air breathed into the nose or mouth. In the workplace, concentration is generally measured in terms of the mass of the hazardous substance per cubic metre of air, for example mg m3, or in parts of the substance by volume per million parts of air, i.e. ppm. However, the units of exposure should really be something like mg m3 h, i.e. the product of intensity and duration, but it is conventional to express exposures in units of concentration only and to make the duration a standard period, normally either 8 h or 15 min. So for example, a painter exposed to 10 mg m3 of toluene for 4 h out of an 8-h shift would have an exposure of 40 mg m3 h but we would normally say that the painter’s 8-h average exposure was 5 mg m3, i.e. 40/8 = 5. If the exposure is to be assessed over a 15-min period then the sample is normally collected for this duration. In the case of our painter, the 15-min exposure might be measured on a number of occasions partway through the shift when it was judged the exposure level was highest.

If exposure is measured by collecting a number of samples over an 8-h period then it is conventional to take the average of these, allowing for the different duration of each sample -this is called a time-weighted average (TWA). For example, we could have measured the painter’s exposure during the first hour when she was preparing the job, for the next 3 h when she was painting and then for the final 4 h when she was doing paperwork in the office. The results from this monitoring are shown in .

The 8-h TWA exposure level (E8-h TWA) is calculated by multiplying each exposure level in the table by the corresponding duration, summing them all and then dividing by the total duration (i.e. 8 h). Mathematically this is written as


where Ei are the exposure levels, ti the durations and n the number of samples collected over the 8 h. If you calculate the 8-h TWA for the results in the table you will see that it is also 5 mg m3, i.e. (4 × 1 + 12 × 3 + 0 × 4)/8 = 40/8 = 5.

For hazardous substances, exposure may occur by several different routes; i.e. the substance may enter the body by inhalation, by ingestion, by injection or by passing through the skin. For most chemicals the science was originally developed to address problems from inhalation, principally because this was considered the most important route for the majority of substances. This means that for the other routes of exposure the measurement methodologies and the concept of an OEL are less coherently developed. Also, for physical agents there are differences in the units of measurement and in the approaches used to obtain a measure of exposure, but again these issues are dealt with in the later sections of this book.

Toluene exposure measurements for a painter

Activity Duration of sample (h) Exposure level (mg m3)
Preparation 1 4
Painting 3 12
Paperwork 4 0

1.2 Hazard and risk

We saw earlier that a hazard is a situation that has the potential to cause harm to a person, e.g. exposure to toxic chemicals, absorption of energy transmitted as microwaves or exposure to loud noise. Risk is the possibility of that hazard causing harm to a particular individual or group of individuals in a given time period. Hazard and risk can be expressed in words, numbers or any other way, as long the information is meaningful. However, it should be clear that if you cannot come into contact with a hazard, i.e. you are not exposed, then the risk is zero.

The type of harm that may occur is an important aspect of a hazard. As each hazard may cause a range of harm, from minor injury to death, the type of harm must be specified. For example, exposure to a substance may carry a risk of causing respiratory tract irritation or cancer, and it is obvious that the latter consequence would be the more serious outcome. This is partly because cancer is a life-threatening illness and irritation is generally a nuisance, and partly because once the cancer has been initiated it tends to be irreversible whereas the irritation will mostly cease soon after exposure has ended.

In assessing risk, care should be taken to ensure that possible accidental exposures are properly considered and there is not just a focus on routine conditions. Accidental exposures may include spillages, activities such as cleaning and maintenance, which can disturb deposited material making a hazardous substance airborne again, entry into confined spaces that contain hazardous substances without proper protective equipment, and wearing contaminated personal protective clothing. Rules that prohibit eating, drinking, smoking, applying cosmetics, nail biting and so on can help prevent substances hazardous to health entering the body by inadvertent ingestion. Advice about washing and showering can reduce the risk from dermal exposure.

1.3 Risk assessment

Risk assessment is the process of making decisions about the acceptability of the risk and the need to take precautionary or protective measures. Risk assessments are part of the wider systems designed to ensure effective management of health and safety in the workplace, but they are a key component that is necessary for the protection of health.

The outcome of a workplace risk assessment is usually expressed in terms such as the risks are acceptable, the risks are unacceptable or the outcome of the risk assessment is uncertain and further information is required to arrive at an appropriate conclusion. One would generally start by making a qualitative assessment of the risks by using all of the relevant available information. For example, if a laboratory technician is using a very small quantity of a low-toxicity liquid in a fume cupboard while wearing appropriate protective gloves then it will almost certainly be the case that the risks from handling the chemical are acceptable, and any further investigation would be unnecessary. In contrast, people working in a factory where the noise levels were so loud that you cannot hear someone next to you talking are probably exposed to unacceptable risks to their hearing and the most important thing to do is ensure that a suitable noise control strategy is implemented as soon as possible. However, it may be necessary to obtain some simple quantitative data to arrive at a satisfactory conclusion or it may be that measurements of exposure are needed to convince management that the risks are unacceptable and that they need to take action. This type of data may be obtained by measuring exposure.

Any measurements need to be interpreted in terms of risk and for most situations the OEL provides the best way of doing this; exposures above the limit being unacceptable. Where there are no published OELs then a more considered approach is needed to evaluate the risks and in these circumstances it will be necessary to seek the advice of an occupational hygienist or other relevant scientific or medical expert.

1.4 The stages of a risk assessment

1.4.1 Identify the hazard

Workplace risk assessments are best carried out with the cooperation of the relevant managers and workers. They will have access to the sort of information that is needed to come up with an appropriate conclusion and they will also be able to help identify possible solutions where the risks are considered unacceptable.

The first stage in risk assessment is to define the scope and state which area, task or activity is to be assessed. You may choose to start by drawing a hazard map of the area, or a flow diagram of the tasks or activities to be assessed. Check that you have covered all relevant activities including cleaning, breakdowns and maintenance. Then list the physical, chemical or biological hazards associated with each activity, taking care to consider process-generated agents and remembering to identify accidental exposures. Examine the workplace layout and the process to identify where different hazardous agents may interact and make a note of the environmental conditions such as air temperature, humidity and general ventilation. It is often useful to seek out published information from trade associations, regulators, e.g. the Health and Safety Executive (HSE) or other reputable information sources.

Risks that are reasonably foreseeable should be noted and some preliminary decisions taken on their significance. The risks judged to be insignificant should be noted to indicate that they have been considered but the significant ones will require more attention.

1.4.2 Decide who might be affected and how

List all types of personnel who may be exposed to the hazards, including how many people are in each group, the split by gender and any other relevant demographic information. Some workers such as new and young workers, new or expectant mothers and people with disabilities may be at particular risk and there may be specific legal obligations in respect of them.

It is usual to identify work groups or types of employees such as process workers, maintenance workers, welders, office workers and so on, and nonemployees such as contractors, customers or neighbours. If appropriate, you should also record more specific job titles represented in each group.

1.4.3 Evaluate the risks

Identify the possible types of harm that could be realised, for example whether there is a chance of acute or chronic effects. Then make an estimate of the exposure that the identified groups of workers and non-workers may experience. The sorts of things that need to be considered here are the following:

In deciding whether there is adequate control, you should also consider the principles of good practice for the control of exposure to substances hazardous to health as laid down in the COSHH regulations. These principles, which can be extended to physical agents, are the following:

). This type of information provides a guide as to the level of control that is needed to ensure the risks are properly controlled.

1.4.5 Record the significant findings

At the end of the risk assessment process, you must be in a position to demonstrate that the assessment was suitable and sufficient. In particular, that:

If the current controls are insufficient to properly control the risk then further measures must be introduced. Any recommendations for further control measures should be identified in the written record of the assessment.

In our view, it is good practice to always record a risk assessment, although if there is a small number of people exposed and the hazard is relatively slight then it may not be necessary to make an extensive written record of the assessment.

1.4.6 Review the assessment regularly and revise it if necessary

Risk assessments must be reviewed at suitable intervals or if there has been an important change to the work process or work environment. If any cases of injury and ill health are identified amongst workers on a process then the risk assessment should be immediately reviewed to ensure that it is still valid.

1.5 Who should carry out risk assessment

To undertake a risk assessment you need to have an appropriate level of training, knowledge and experience. Some regulations or official guidance specify that the risk assessment should be undertaken by a competent person, but you should assume that competence is required by everyone undertaking this type of work. However, the level of competence may vary from one situation to another, and for example someone assessing the risk in a large petrochemical site will need to have a greater level of competence than another individual assessing the risk for an office environment. A competent person must be able to recognise their own limitations and when they need further assistance from someone more experienced. This may involve assistance from within the company or external advisors, experts or consultants. Usually, larger companies have specialists such as safety professionals, occupational hygienists and engineers to assist; smaller organisations will need to outsource such advice.

References and further reading

Sadhra SS. (2005). Principles of risk assessment. In: Occupational Hygiene, 3rd edition (Gardiner K, Harrington, JM, eds). Oxford, UK: Blackwell Publishing.

HSE (1999). Five Steps to Risk Assessment. INDG163. Sudbury, UK: HSE Books. Available at .

HSE (2004). A Step by Step Guide to COSHH Assessment. HSG 97. Sudbury, UK: HSE Books.

HSE (2003). COSHH Essentials Easy Steps to Control Chemicals. HSG 193. Sudbury, UK: HSE Books.

The Scientific Committee on Occupational Exposure Limits (SCOEL) provides scientific advice to the European Commission to underpin regulatory proposals on exposure limits for chemicals in the workplace. Available at .

More information on good control practice techniques is available at HSE’s COSHH Essentials website. .

HSE’s COSHH website provides information about the regulations and how to comply with them, plus a list of WELs. .

HSE information about the calculation of exposure with regard to the specified reference periods, with examples. Available at .