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Assessing and Controlling Risks from Workplace Exposure to Chemicals. M. D. Topping, Health Directorate, Health and Safety Executive, London, UK
INTRODUCTION
Chemicals have brought many benefits to society and hundreds are in use involving a wide range of industrial processes. However, many chemicals have the potential to cause adverse health effects if absorbed into the body although the amount needed to cause ill health can vary by orders of magnitude. The potential effects range from mild irritation of the airways to cancer. The challenge is to use chemicals to maximum social and economic benefit while protecting workers and the public. This paper describes how risks to workers' health are assessed and controlled within the UK and the increasing harmonisation of chemical risk assessments within the EU.
ASSESSING THE RISKS
The first step in assessing the risk posed by a chemical is to determine the hazards (that is, the adverse effects on health the chemical has the inherent capacity to produce) by carrying out a comprehensive and critical review of all the toxicological data relevant to human health. This generally includes consideration of acute and chronic toxicity, skin, eye and respiratory irritation and sensitisation, mutagenicity (i.e. the ability to damage genetic material), carcinogenicity and the ability to cause effects on reproduction. It is also necessary to review information on the physicochemical properties of the substance, measurement methods, uses and any information on exposure levels among working populations.
The next step is to consider whether a no observed adverse effect level (NOAEL) can be reliably identified from the toxicological data and if so at what level. This is the highest exposure level at which across all the studies that have been conducted on the substance in question, no significant health effect has been seen. Frequently NOAELs are determined from studies carried out in experimental animals and for different substances the data will vary in both quality and quantity. Thus to arrive at a level at which it is judged there is no significant risk to the health of a working population it is normally necessary to apply an uncertainty factor to the NOAEL. The size of this uncertainty factor will depend on the amount and quality of the toxicological data, the seriousness of the adverse effect and whether human data is available. The uncertainty factors used by the UK range from 1 for substances for which the critical effect is sensory irritation and reliable data are available from human studies to around 50 for substances for which the critical effect is an adverse effect on fetal development in animals.1
For some substances NOAELs cannot be identified. This is either because the toxicological data is insufficient, particularly at low doses, to identify a no-effect level although one may in fact exist, or because the chemical is considered to pose a risk at all levels of exposure to which the substance can be realistically controlled. Our recent experience within the UK is that respiratory sensitisers fall into the first category and the latter is dominated by genotoxic carcinogens. These are carcinogens which are considered to exert their carcinogenicity by causing direct damage to genetic material. It is not clear whether or not a dose threshold exists for such activity and in view of this uncertainty and the strong concern surrounding this type of toxicity it is considered prudent to assume that there is no discernible threshold and to regulate the substance as though any level of exposure carries with it some risk of producing cancer.2
MANAGING TBE RISKS
The UK legal system
Within the UK the Control of Substances Hazardous to Health Regulations (1994), set under the Health and Safety at Work Act (1974), provide the legal framework for the managing the control of risk to health from hazardous substances. COSHH provides a goal setting legislative framework which requires employers to assess and manage risks arising from the use of chemicals in the workplace. A key requirement is that employers must ensure that the exposure of their employees to substances hazardous to health is either prevented or, where this is not reasonably practicable, adequately controlled. Under the COSHH Regulations two types of occupational exposure limit are used to define adequate control by inhalation, these are the occupational exposure standard (OES) and the maximum exposure limit (MEL). Both are important risk management tools. For an OES control is considered adequate if the OES is not exceeded or if it is exceeded the employer identifies the reasons and takes appropriate action to remedy the situation as soon as is reasonably practicable. In contrast, for an MEL exposure is only adequate if the level of exposure is reduced so far as is reasonably practicable and in any case below the MEL.
These two types of limits are applied to the two outcomes of the risk assessment. OESs are established for substances for which a level can be identified at which there is considered to be no significant risk to human health, providing compliance by industry with value identified by the risk assessment is reasonably practicable. MELs are considered for substances for which such a NOAEL cannot be identified or compliance with the "safe" level of exposure identified by the risk assessment is not reasonably practicable.
Tolerability of risk
HSE first introduced the concept of tolerability of risk in
the context of the assessment of risks from nuclear power
stations.3 The risk triangle is shown in Figure 1. In
essence it is divided into 3 regions. At the bottom the risks are
considered to be negligible and therefore there is no need for
control measures to further reduce them. The top line represents
the maximum tolerable level and any further increase in risks is
considered unacceptable and cannot be justified except in
extraordinary circumstances. The area in between is known as the
"as low as is reasonably practicable" (ALARP) region.
In this region risks are only considered tolerable if risk
reduction is impracticable or if its cost is grossly
disproportionate to the improvement gained.
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OELs and tolerability of risk
Since OESs are set at a level at which there is judged to be no significant risk to health, then they are at the negligible risk line in Figure 1. The excursions permitted above OES values are in the ALARP region, consistent with the requirement on employers to reduce exposure to the OES as soon as is reasonably practicable.
For substances for which a negligible risk level cannot be identified how is an OEL to be determined? One option is to estimate a negligible risk level using models to extrapolate from the high doses seen in animal experimentation to the much lower doses relevant to the workplace. There a number of such models available. The drawback is that none have been validated and depending on the one used, risk estimates can vary by orders of magnitude.2 The UK do not use this type of approach to defining risk.
In the absence of numerical risk estimates, the approach used to set OELs for substances for which an NOAEL cannot be identified is to set MELs at a level that can be achieved with good occupational hygiene practice. This involves taking into account any evidence available on the level of risk posed by the substance, although more often than not such evidence is sparse. In view of the serious nature of health effects of substances considered for a MEL, once a level achievable with good occupational hygiene practice has been defined higher exposure is intolerable. Thus a MEL sits on the maximum tolerable line in Figure I for that particular substance. Since we do not have numerical estimates of risk, the actual risk at that level will vary between substances. The requirement on employers to reduce exposures as far below the MEL as is reasonably practicable, is consistent with the need to reduce exposure through the ALARP region, providing the costs are not disproportionate to the benefits.
Gaining acceptance of proposals for OELs
The Health and Safety Commission (HSC) is responsible, in the UK, for establishing standards for health and safety, subject to approval by parliament. HSC is a tripartite body with representatives from employers, trade unions and independents. AD decisions are consensual, so proposals for OELs have the full support of both sides of industry. In addition each proposal is subject to public consultation, which includes publication of the complete risk assessment and the reasoning behind the type of limit proposed (i.e. OES or a MEL) and the numerical value. For MELs a cost benefit assessment is available which gives an estimate of the costs to industry of complying with the limit and the benefits in financial terms, based on a protocol for estimating the nominal number of cases of ill health the limit might prevent.
Other risk management approaches
In general our preference is for use of risk management regimes to allow substances to be used safely, however the risks posed by a few substances are so severe that they have been banned completely in the EU, for example crocidolite asbestos.
THE EUROPEAN DIMENSION
In 1993 the EU Existing Substances Regulation came into force. This provided a community system for the assessment of risks from priority chemicals to ensure better management of risks within the framework of Community provisions. The risk assessments cover human health, including workers, consumers and threats to public health via the environment as well as environmental concerns. Detailed guidance on the format and scope of the assessments has been produced by the EU, thus providing a common format. The first tranche of risk assessments, which are being carried out by a number of Member States, are nearing completion. The risk assessments will be considered and outcomes agreed by a committee of representatives from Member States. One possible conclusion is the need for risk reduction measures. Where this applies to worker exposure there is the possibility of establishing OELs at the community level or the risk assessments can be used by Member States for the establishment of limits within domestic legislation. A standard approach and format for chemical risk assessments offers the opportunity for considerable progress on management of risks from chemicals since Member States should be able to develop confidence in each other documents, thus eliminating the need to duplicate effort.
CONCLUSION
The paper shows that the UK regulatory approach to assessing and controlling risks from chemicals is based on a thorough and rigorous assessment of all the toxicological data, a robust goal setting legislative framework for controlling risks and acceptance of proposals by extensive consultation with all interested parties.
REFERENCES