Target audience

The Workplace Exposure Standards (WES) are intended to be used as guidelines for health risk management.

PCBUs and people with duties under the Health and Safety at Work Act 2015 (HSWA) and the Hazardous Substances and New Organisms Act 1996 (HSNO Act) may use this page as a reference; but it is recommended that specialist advice is sought prior to engaging in monitoring programmes or exposure control.

It is not recommended that untrained persons use WES to assess health risks. Professional judgement is required in making decisions regarding safe levels of exposure to chemicals in the workplace.

WES, BEI and PES explained

WES, and Biological Exposure Indices (BEI) are health-based values. This means they are based on minimising health risk and don't take into consideration practicability of achieving or measuring the value.

We consider it critical to set health-based values as risk criteria, so that risk assessment is based on an actual understanding of health risk. Efforts need to be directed to the adequacy of controls rather than merely measuring a level of exposure.

What is a WES?

A WES is a guidance value that refers to the airborne concentration of a substance at which it is believed that nearly all workers can be repeatedly exposed day after day without coming to harm. WES are provided by WorkSafe as a tool in risk management. They are intended to be used as risk criteria for health risk assessment

What is a BEI?

A BEI is guidance value for assessing biological exposure monitoring results and is another tool that can be used in managing risks to health and safety associated with substances hazardous to health in the workplace.

What is a PES?

A prescribed exposure standard (PES) is a prescribed workplace exposure standard or biological exposure index that has the purpose of protecting persons in a workplace from harm to health. PES must be complied with (and is not merely a guidance value like WES). PES are prescribed in:

  • regulations;
  • a safe work instrument; or
  • the Hazardous Substances and New Organisms Act 1996 as a control under section 77 or 77A, or an exposure limit under section 77B or a group standard approval issued under section 96B.


Defining an exposure level that will achieve freedom from adverse health effects is the major consideration for assigning these WES. However, merely demonstrating that exposures are unlikely to exceed the WES value does not guarantee that all workers are protected from discomfort or ill-health. The range of individual susceptibility to hazardous and toxic substances is wide, and it is possible that some workers may experience discomfort or develop occupational illness from exposure at levels below the WES.

The numerical value of two or more WES must not be used to directly compare the relative toxicity of different substances as the biological potency and toxicologic effects used to derive a WES are specific to each substance.

When interpreting the risk posed by individual substances, the documentation that supports the WES should be consulted.

When applying these WES values, it is important to understand the end-point health effects for which it is designed to protect for, and the limitations of the WES or data used to derive the value. It is good practice to consider WES values from other organisations in managing the health risk. Relevant sources of other exposure standards include the Gestis substance database(external link), the ACGIH®(external link), SCOEL, ECHA(external link), DFG(external link), DECOS, and Safe Work Australia(external link). Access to their documentation that explains the value is necessary, rather than just adopting a number without any information as to how and why it was selected.

Substances without a WES

In many cases well-documented data exist to help determine WES. But for some substances, the available toxicological and industrial hygiene information is insufficient to enable highly reliable standard-setting. As such some substances do not have WES.

If a substance doesn’t have a WES, this should not be taken to mean that it is safe under all conditions, and that no restriction should be placed on its use.

Regardless of the substance, it is important to eliminate or minimise the concentration of airborne substances as far as is reasonably practicable.

Substances with a WES-TWA, but without a WES-STEL

To provide an upper limit on short-term exposures, an excursion limit (EL) may be applied for substances that have a WES-TWA, but no WES-STEL or WES-Ceiling.

Before applying an EL, further information should be obtained to help inform whether or not doing so is an appropriate approach, rather than assuming it to be appropriate for all substances. Such information may include acute toxicological data or the existence of short-term exposure limits from other jurisdictions.

Applying WES

Personal sampling

Monitoring workers’ exposure will involve comparison of results against Workplace Exposure Standards and Biological Exposure Indices.

Workplace exposure standards (WES) are values that refer to the airborne concentration of substances at which it is believed that nearly all workers can be repeatedly exposed day after day without coming to harm. The values are normally calculated on work schedules of five shifts of eight hours duration over a 40-hour work week.

In all instances, workplace exposure standards relate to exposure that has been measured by personal monitoring using procedures that gather air samples in the worker’s breathing zone. The breathing zone is defined as a hemisphere of 300mm radius extending in front of the face and measured from the midpoint of an imaginary line joining the ears.

Substances with multiple WES (for different periods of exposure) will require monitoring for those specific periods. For example if a substance has a WES- TWA (time weighted average) then exposure for the whole shift needs to be assessed. This does not necessarily mean exposure has to be measured over the whole shift, but if exposure will vary, full shift sampling will provide the most useful data for the risk assessment. If the substance also has a WES-STEL (short term exposure limit), exposure over 15-minutes needs to be assessed.

It is important to ensure results are measured and calculated over appropriate time frames when comparing to a specific WES, and that WES are adjusted accordingly for extended work shifts. See Adjustment of WES for extended work shifts

The numerical value of two or more WES must not be used to directly compare the relative toxicity of different substances. Apart from any inconsistency that may result from the information that was available at the time each WES was set, the biological basis for assigning the WES varies. Some WES are designed to prevent the development of ill health after long-term exposure (WES-TWA), others to reduce the possibility of acute effects (WES-Ceiling, WES-EL, WES-STEL).

Assessing exposure

Assessing workers’ exposure relies on good sampling strategy in addition to the correct sampling equipment and interpretation of results.

WorkSafe NZ recommends that professional help be sought in the development and implementation of a sampling strategy and interpretation of results (for example, from an appropriately qualified occupational hygienist).

When carrying out exposure assessments, assessing health risks, or assessing the need for, or effectiveness of controls, the assessor should have competence in:

  • the risk assessment process
  • the tasks, processes or exposures being assessed development of sampling strategy
  • selection and use of sampling equipment and sampling media sampling methods
  • interpretation of data
  • criteria on which WES are based
  • relevance and application of statistical analysis of exposure data.

Assessor competency should be maintained by subscribing to a programme of continuous professional development. Such programmes are available to members of professional bodies such as the New Zealand Occupational Hygiene Society (NZOHS)(external link).

An assessor could equally develop their own programme that covers on-going training (including refresher training), training or recruitment that addresses lack of competence in a particular areas, attendance at conferences, meetings or webinars etc.

Assessors not yet fully competent to operate independently should consider being mentored by a fully competent assessor such as a full member of the NZOHS. Mentoring involves meetings between the mentor and mentee that take the form of a professional discussion around personal development, current projects and the challenges faced. One of the aims is for the mentor to get the mentee to think about how they might approach a problem, what other things they might encounter and how they might deal with them. Mentoring arrangements should be documented to help ensure their effectiveness.

WorkSafe encourages PCBUs to use the services of consultants who are listed on the HASANZ Register(external link).

HASANZ is the Health and Safety Association of New Zealand and is the umbrella organisation representing workplace health and safety professions in New Zealand. The register lists independent consultants and in-house professionals – generalists and specialists – who meet the competency standards of an association that is a full member of HASANZ. For those offering occupational hygiene services, their association is the New Zealand Occupational Hygiene Society (NZOHS).

By selecting a consultant from the HASANZ Register, a PCBU can have confidence that they are selecting a person who is competent to undertake the services for which they are listed.

Good communication skills, as well as the systematic collection of data and information are essential and reports should present the results and any recommendations clearly and in a style that the PCBU will understand.

The assessor must have a clear understanding of the limitations of their own competencies.

Sampling strategy

Sampling strategy will usually include identifying groups of workers for whom risk and exposure profiles are similar. These groups are called SEGs (similar exposure groups). Choosing a representative unbiased subsample of the SEG should be sufficient for assessing exposure and risk for the whole SEG.

Most worker exposure monitoring will be occasional in that the workers will not wear monitoring equipment all the time (with some exceptions (for example, explosive gas meters), which are usually used for safety risk management rather than health risk). The regularity of worker exposure monitoring will depend on the objectives and outcomes of the risk identification and analysis. For example, if the risk identification or analysis indicates that exposure can vary considerably from day to day, then monitoring may need to occur on a more regular basis than an exposure that does not change considerably over time, or an exposure that is well managed.

Monitoring should occur when there are any changes in processes or activities that result in, or may result in, a change to exposure, or if it is not certain whether or not the airborne concentration exceeds the Workplace Exposure Standard (WES) or presents a health risk.

Variation in exposure

Exposure levels are commonly variable, even in work that is regular and consistent. Variation in worker exposure arises from variation in work activities, control methods and environmental conditions.

Due to this variation, exposure measured on a single day may not reflect exposure on other days. Even samples from multiple days may not reflect the true variation in exposure that may occur over the long term. With this in mind, the monitoring strategy must be designed to provide sufficient measurements to reflect the risk to the worker from the variation in exposure.

It is very rare for all exposures for a worker to be measured all the time. Frequently only one or two shifts will be sampled and this data will be used to make judgements about exposures over many months or years. If the worker is exposed every day for five years, and their exposure is assessed once a year, then five days of data is being used to make judgements about 1250 days of exposure. Various methods are available for determining an appropriate number of samples to account for variation. Methods include:

  • at least one employee in five from a properly selected SEG (UK Health and Safety Executive HSG173 (2006)1

  • a calculated number of samples based on previous data, using t-statistics and co-efficient of variation (source W501 OH Learning, Measurement of Hazardous Substances, 2009)2

  • methods of Rappaport, Selvin and Roach (1987) based on the number of samples needed to test the mean exposure of a lognormal distribution of exposures against an exposure standard (source W501 OH Learning, Measurement of Hazardous Substances, 2009)2

  • South African Mines Occupational Hygiene Programme – sample 5% of workers in an SEG3

  • American Industrial Hygiene Association suggests 6–10 samples are sufficient to give a picture of an exposure profile. In respect to the minimum number of samples to be collected, fewer than six samples in any one SEG leaves a great deal of uncertainty about the exposure profile (AIHA 2015)4

  • European Standard EN 689:2018 ‘Workplace exposure – Measurement of exposure by inhalation to chemical agents – Strategy for testing compliance with occupational exposure limits’.

Statistical analysis of sampling results

Multiple exposure samples generally allow for better understanding of the variation in exposure, and thus provide more detailed information for the risk assessment.

Where multiple samples are taken, application of appropriate statistical analysis to sampling results can be valuable in:

  • assessing confidence that the measured results represent the ‘true’ exposure profile (the profile you would see if you were to measure the exposure every shift, and you were to measure all workers in the SEG)
  • interpreting whether exposures are likely to exceed the WES
  • managing uncertainties in exposure assessment and health risk assessment.

Application of appropriate statistical analysis to sampling results is important in order to assess how closely the results represent the ‘true’ exposure profile and can be used to assess the likelihood that workers in that similar exposure group will exceed the designated WES, how close the true exposure is to the WES and assist in determining if there is risk that needs further control.

Useful resources for statistical analysis of occupational hygiene samples include:

Using WES to assess risk

When evaluating exposure in relation to a WES, the following points must be considered:

  • How representative is the sampling programme in regard to variation in exposure, and how do the results represent the ‘true’ exposure profile?
  • Variability of exposure means that occasional high results can occur even where the exposure is generally well controlled.
  • Comparison of exposure monitoring data with the designated WES level does not guarantee that all workers are protected from discomfort or ill health due to individual susceptibility.

The above considerations show that using exposure monitoring data to assess health risk is rarely a straightforward process of comparing a sample result, or an average, to a WES.

Various organisations have developed guidelines to address the issue of how to assess health risk using a WES to determine whether further control of exposure needs to occur.

Care should be taken to ensure the sampling strategy and subsequent analysis follow the selected method closely as they all have conditions that must be met and can use slightly different statistical criteria.  Examples of these methods and guides are:

Adjustment of WES for extended work shifts

Workplace Exposure Standard Time Weighted Averages (WES-TWA) are derived on an eight hour work day and 40 hour work week. When shifts are longer than this, either over a day or a week, the WES-TWA needs to be adjusted to account for the longer period of exposure and shorter recovery time.

Various models are available to make the adjustment and each may result in a different adjusted WES.

The selection of an appropriate model is dependent on various factors such as: ease of use; availability of an adjustment model for a specific WES; and the availability of relevant toxicology and pharmacokinetics data for pharmacokinetic models.

A useful document for discussion on adjustment models is the Australian Institute of Occupational Hygienists’ Position Paper on ‘Adjustment of Workplace Exposure Standards for Extended Workshifts’(external link)

A simple method to use is the Brief and Scala Model. A criticism of the model is that it is generally considered to be excessively protective for some substances. Other models include web based tools such as the IRSST ‘Quebec’ model. A summary of these models is given below.

When a WES-Ceiling or WES-STEL has been assigned, no correction for shift patterns is required. The exposure level for the appropriate period (instant or 15 minutes) is compared directly with the Ceiling or STEL.


An adjustment is made to the WES by applying the following formula:

Daily exposure adjustment:

Where h = hours worked per day

Formula for daily exposure adjustment: Adjusted WES-TWA equals open bracket 40 multiplied by open bracket 168 minus h close bracket multiplied by WES-TWA close bracket divided by open bracket 128 multiplied by h close bracket

Seven day work week adjustment:

Where h = hours worked per week

Formula for seven-day week adjustment: Adjusted WES-TWA equals open bracket 40 multiplied by open bracket 168 minus h close bracket multiplied by WES-TWA close bracket divided by open bracket 128 multiplied by h close bracket


Example of adjusting for an extended work shift using the Brief and Scala model

Substance: Isopropyl alcohol – WES-TWA: 400ppm, WES-STEL: 500ppm

Work shift: 12 hours

Formula showing an example of adjusting for extended work shifts: open bracket 8 multiplied by open bracket 24 minus 12 close bracket multiplied by 400ppm close bracket divided by open bracket 16 multiplied by 12 close bracket equals 200ppm 12 hour

The average exposure over the 12-hour shift would be compared with the 12-hour WES-TWA standard of 200ppm. No adjustment is required for the WES-STEL.


The Quebec Institut de Recherche Robert-Sauve en Sante et en Securite du Travail (IRSST) has developed a computer-based tool to calculate an adjusted TWA. The model makes adjustments of the Quebec WES (called PEVs) as defined in the Quebec Regulation Respecting Occupational Health and Safety (RROHS).

Some of the Quebec WES differ from New Zealand. Although adjustment factors are provided in the model, these are based on the critical health endpoints used to derive their WES. Before applying the Quebec Model it should be confirmed that the adjustment classification in the Quebec Model for the selected substance is appropriate for the health endpoint(s) used to select the New Zealand WES. The model is available at IRSST(external link)


The AIOH have developed the WES Shift Adjustment Tool which has been based on the well accepted technical information currently available. The tool allows shift adjustment based on either the Brief and Scala Model or the Quebec Model depending on how well hours worked and shift patterns are understood. The AIOH tool applies the Quebec Model using the critical health endpoints for the Australian WES. Therefore, it should be confirmed that the health endpoint(s) used to select the Australian WES is the same as the New Zealand WES before applying the adjustment factor to the New Zealand WES. The tool is available at: AIOH(external link)


In this guideline various exposure reduction factors are applied depending on the timeframe for response (immediate, medium or long term), health effect (acute, chronic, irritation, narcosis) and shift length.

The appropriate reduction factor is selected and applied to the WES. The model is available at AIOH(external link)


There are a number of pharmacokinetic models in use. These models are based on the concept of body burden and how the biological half-life of a substance can have a significant impact on the maximum body burden for a given work schedule. They are based on ensuring that the maximum body burden for an extended work shift doesn’t exceed that for an eight hour shift.

These models are generally considered more accurate however, they can be very complicated and, as half- lives can vary substantially between different individuals, there are limitations.

Mixed exposures

Generally, WES are listed for a single substance or a range of compounds. In some instances, a WES has been set for a group of substances (for example, petrol vapours).

Often a worker will be exposed to several substances over the working day. Before an assessment of the existing hazards can be made, it is important to determine the airborne concentration of each substance.

Independent effects

If there is evidence to suggest that the actions of hazardous/toxic substances on the body are independent, the concentrations of each individual substance should be compared directly with its own WES value (- TWA, -STEL, or –Ceiling as appropriate).

This is most obvious when two (or more) substances have different toxic actions, and cause adverse effects on different target organs. An understanding of the health basis on which the WES has been set is essential for determining if the substances have independent health effects.

An example is toluene-2,4-diisocyanate and toluene. The toluene-2,4-diisocyanate WES is based on minimising the potential for respiratory tract effects and sensitisation. The toluene WES is based on minimising the potential for central nervous system depression.

Additive effects

If two or more hazardous substances have similar toxicological effects on the same target organ or system, their combined effect should be considered. In this case the combined exposures need to be compared against the WES of the mixture, as well as each individual substance against its specific WES.

Greater than additive effects

The combined action may be greater than that predicted from the sum of the individual responses (synergistic effect), or a substance that is not itself toxic could enhance the effect of a toxic substance.

The present understanding of synergistic effects is far from complete, and emphasises the need for a prudent approach to be taken with mixed exposures. It is important that the assessment of all exposures should be made in consultation with suitably qualified and experienced persons; especially so with mixed exposures.


Aerosols encountered in the workplace include airborne particulates (this includes dusts and fumes) and mists.

Dusts are discrete particles suspended in air, originating from the attrition of solids or the stirring up of powders or other finely divided materials. Dusts encountered in the workplace typically contain particles covering a wide range of sizes.

Fumes are very small airborne solid particulates with diameters generally less than 1µm. They may be formed by both thermal mechanisms (for example, condensation of volatilised solids, or incomplete combustion) and chemical processes (for example, vapour phase reactions). Agglomeration of fume particles may occur, resulting in the formation of much larger particles.

Mists are droplets of liquid suspended in air. They may be formed by the condensation of a vapour, or by mechanical actions such as the atomisation of liquids in spray systems.

Aerodynamic equiavalent diameter (AED)

A parameter used to predict the likely behaviour of a particle in air is its Aerodynamic Equivalent Diameter (AED). The aerodynamic equivalent diameter of a particle of any shape and density is defined as the diameter of a sphere with a density of 1.0g/cm3 which has the same terminal velocity of settling in still or laminarly flowing air as the particle in question.

Health effects of particulates

Airborne particulates are associated with a variety of adverse health effects and may have one or more of the following properties:

  • infectious
  • carcinogenic
  • fibrogenic
  • allergenic
  • irritative.

The total concentration of the substance in air, either in terms of the weight or number of particles per unit volume, is not the only factor influencing its toxic potential. The toxic potential of a substance is influenced by a number of factors including concentration, particle size, mass, surface area and solubility.

Inhalable and respirable dust

Inhalable dust is the portion (or fraction) of airborne dust that is taken in through the mouth and nose during breathing.

Respirable dust corresponds to the fraction of total inhalable dust that is able to penetrate and deposit in the lower bronchioles and alveolar region.

Unless otherwise stated, the WES for dusts refers to inhalable dust. The WES that apply to particulates not otherwise classified apply to particulates that (i) do not have a specified WES, (ii) are insoluble or poorly soluble in water (or, preferably, in aqueous lung fluid if data are available), and (iii) have low toxicity (that is, are not cytotoxic, genotoxic, or otherwise chemically reactive with lung tissue, and do not emit ionising radiation, cause immune sensitisation, or cause toxic effects other than by inflammation or the mechanism of ‘lung overload’).

Even biologically inert, insoluble, or poorly soluble particulates may have adverse effects and it is recommended that airborne concentrations should be kept below 3mg/m3 for respirable particulates and 10mg/m3 for inhalable particulates, until such time as a WES is set for a particular substance.

Inhalable dust

Criteria defining inhalable mass fractions have been defined by the International Standards Organisation (ISO). The definitions describe collection efficiency curves that pass through the following points:

Collection efficiency curve for inhalable dust







% inhalable mass fraction







Where d is the equivalent aerodynamic diameter of the particle in μm.

Different types of sampling devices that are specifically designed to conform to this specification may provide conflicting results if a significant proportion of the particles are larger than approximately 30μm.

At present there is no one acceptable procedure for obtaining a sample that accurately reflects the inhalable mass fraction (under various environmental conditions). However, for the purpose of these standards, the inhalable dust is to be collected according to the method set out in AS 3640–2009: Workplace Atmospheres – Method for Sampling and Gravimetric Determination of Inhalable Dust7

The use of either of two personal sampling heads is recommended: the United Kingdom Atomic Energy Authority (UKAEA) sampling head or the IOM inhalable dust sampling head developed by the UK Institute of Occupational Medicine, Edinburgh.

Respirable dust

Respirable dust is the proportion of airborne particulate matter that penetrates to the unciliated airways when inhaled. Respirable dust samples are to be collected according to the method set out in the Standards Australia publication AS 2985–2009: Workplace Atmospheres – Method for Sampling and Gravimetric Determination of Respirable Dust8

Care is advised in the selection of cyclone sampling heads used for the determination of respirable dust. Recent research indicates that oversampling may occur with some sampling devices used at the historically recommended flow rates. It is strongly recommended that hygienists conducting this work obtain advice from the manufacturers or suppliers of such equipment to inform their equipment selection decisions.

This Standard refers to a sampling efficiency curve that passes through the following points:

Collection efficiency curve for respirable dust













Respirability %












Where d is the equivalent aerodynamic diameter of the particle in μm.


An increase in work load can influence the uptake of a substance by increasing the lung ventilation rates and blood flow.

Exposure standards have generally been derived assuming a moderate work load. This factor should be borne in mind, especially where both the work load and exposure are high. The following table presents lung ventilation rates at different work loads. The table can be used:

  1. to indicate if additional care should be taken in interpreting the monitoring results in relation to the WES and
  2. to determine the type and effectiveness of respiratory protection.

Information on the limitations of applying the flow rates is provided in AS/NZS 1715:2009 Selection, Use and Maintenance of Respiratory Protective Equipment. It should be noted that these ventilation rates represent average values and can vary substantially from individual to individual. Current research on values for peak inspiratory air flow indicate that these are underestimated at present.

Lung ventilation rates impacted by workload

Assessment of workload

Average ventilation rate litres/minute

Peak inhalation rate litres/minute 

Low (for example, writing, typing, small bench tool work, standing while drilling or milling small parts)



Moderate (for example, hammering in nails, filing, pneumatic hammering, walking 3.5–5.5km/h)



High (for example, carrying heavy loads, shovelling, digging, pushing or pulling heavy cart, walking 5.5–7.0km/h)



Very high (for example, working with axe, intense shovelling or digging, climbing ladder, stair or ramp, walking in excess of 7km/h)





  1. UK Health and Safety Executive HSG173 Monitoring strategies for toxic substances (2006).
  2. OH Learning W501 Measurement of Hazardous Substances (2009).
  3. South African Mines Occupational Hygiene Programme codebook (SAMOHP) (2002).
  4. The American Industrial Hygiene Association (AIHA) A Strategy for Assessing and Managing Occupational Exposures, 4th edition (2015).
  5. British Occupational Hygiene Society and the Netherlands Occupational Hygiene Society, Testing Compliance with Occupational Exposure Limits for Airborne Substances.
  6. International Council on Mining and Metals (ICMM) Good Practice Guidance on Occupational Health Risk Assessment (2007).
  7. Standards Australia, AS 3640:2009. Workplace Atmospheres: Method for Sampling and Gravimetric Determination of Inhalable Dust. Standards Australia, Sydney, (2009).
  8. Standards Australia, AS 2985:2009. Workplace Atmospheres: Method for Sampling and Gravimetric Determination of Respirable Dust. Standards Australia, Sydney, (2009).