Contents Search publications                                         Previous  Next

MELs and OESs are recommended by the HSC's Advisory Committee on Toxic Substances (ACTS), and its sub-committee, the Working Group on the Assessment of Toxic Chemicals (WATCH). Every year, ACTS and WATCH examine the scientific data on certain chemicals, and the chemicals on their agenda are published in EH40. The agendas and minutes of both committees' meetings are available on the Internet (http://www.open.gov.uk/hse/hsehome.htm). ACTS is a tripartite committee, including members from unions and employers. WATCH consists of technical experts nominated by employers, unions and independents. According to HSE staff, "The tripartite structure is considered important even at this technical level because, with incomplete information and various possible interpretations, a measure of judgement enters into interpretation of scientific data."

WATCH considers whether an OES can be set, and at what level. If WATCH considers an MEL is appropriate, it does not consider the value but refers it to ACTS. ACTS may or may not endorse the OES. If it agrees an MEL is appropriate, ACTS sets the level. Proposals and a summary of the review are published for public comment.

The HSE also uses its National Exposure Database (NEDB) in setting OELs. The NEDB is a database of over 500,000 measurements on 3,000 substances collected by the HSE. Much more data is held by employers and occupational hygienists, which could make the NEDB a much more useful tool, but employers are wary of passing it on to the HSE.

Because there is relatively little good data on the health effects of chemicals at work, members of WATCH also have to rely on educated guesswork. If information from animal studies is available which shows a no observed adverse effect level, WATCH uses this as the basis of the OES and adds an Uncertainty Factor. The size of the Uncertainty Factor depends on the amount of toxicological data, the number of species used in the studies, the routes of exposure studied, the reliability of the studies, the seriousness of the health effect, the slope of the dose-response curve, the workers' age, sex and health, and the cost of the controls.

HSE toxicologist Steven Fairhurst looked at 24 OESs set by WATCH between 1990 and 1993, and found Uncertainty Factors ranging from 1 to 40-60. The substance with the highest Uncertainty Factor was dimethylacetamide, which causes birth defects. Dr. Fairhurst says, "For many substances of relevance occupationally, the toxicological database is rather weak, in both quality and quantity. Critical assessment of each individual original data source has proved essential to ensure accurate portrayal of the study findings. It has also been necessary to exercise considerable predictive and judgmental skills in attempting to construct a coherent and substantial toxicological profile of a substance from the often rather patchy information available. The crucial question is whether or not the Uncertainty Factors applied are correct, such that the OES values confer the desired degree of health protection."

At the end of the deliberations, proposals for new or revised OESs or MELs are published in EH40. Now that COSHH 1999 has come into force, MELs are approved by the Health and Safety Commission and implemented by amending the COSHH Regulations.

"Not only one truth can be interpreted from several scientific data," W. J. Hunter et al., Occupational exposure limits for chemicals in the European Union, Occupational and Environmental Medicine 1997, 54, 217-222.

Chemical Hazard Alert Notices (CHANs) have been introduced by the HSE for substances "where the current scientific information indicates that it is not possible to identify with confidence a level of exposure which is judged to be both safe and realistically achievable." While ACTS considers setting MELs for these substances, CHANs provide interim practical advice to safety reps and employers. A list of CHANs is on the HSE web site (http://www.open.gov.uk/hse/hsehome.htm) and in EH40. When a CHAN is issued, employers should obtain a up-to-date safety data sheet from the supplier or manufacturer.

When WATCH and ACTS recommend a new or revised OEL, a summary of the scientific data they have examined is published by the HSE in an annually updated loose-leaf publication called EH64 - Summary Criteria for Occupational Exposure Limits. The are initially published in draft form to allow trade unions, employers and occupational health professionals to comment on them.

In 1997, the HSE launched a new series of 'risk assessment documents' called EH72. This series supersedes another called EH65. As well as a more detailed version of the WATCH assessment in EH64, EH72 documents also contain information on methods for measuring chemicals in workplace air and on biological monitoring.

Contents Search publications                                         Previous  Next