List of MAK and BAT values 1999 published - New classification for chemical substances in the workplace air

July 21, 1999

The DFG's Senate commission for the investigation of health hazards of chemical compounds in the work area has presented the 1999 list of MAK and BAT values and submitted it to the Federal Ministry for Labour and Social Affairs as an aid to improve industrial safety. The list contains proposed MAK values, i.e. the maximum admissible concentration of a chemical substance present as gas, vapour or aerosol in the workplace air which, according to current knowledge, does not have any adverse effects on the health of the workers concerned even if they are exposed for eight hours daily for a longer period of time. In addition, the substances are classified according to their carcinogenic, genotoxic and sensitising effects as well as their potential to pose a risk to reproduction and to be absorbed percutaneously. Compared with the previous year there were 64 changes and new entries.

16 carcinogenic substances were reviewed and/or classified. It should be noted that this year crystalline silicon dioxide, i.e. quartz, cristobalite and tridymite, were classified as category 1 carcinogenic substances (substances that have been shown to produce cancer in humans).

Epidemiological studies as well as animal experiments have shown that there is a causal relationship between quartz exposure and an increased incidence of lung cancer. Although silicosis points to a high quartz exposure, it is not possible at present to define a scientifically validated limit in order to prevent a health hazard.

Michler's ketone was assigned to category 2, i.e. substances to be considered carcinogenic in humans (as indicated by animal experiments). Tetrahydrofuran and cresol, solvents that are potential tumour-inducing substances, were placed in category 3 (substances suspected of having a carcinogenic potential), and so were metallic mercury and its inorganic compounds after its organic compounds had already been placed in this category earlier. Neutralised cross-linked polyacrylic acid ("superabsorber"), sulphuric acid, 2,3,7,8-TCDD and chloroform were placed in the newly created category 4. Since it has to be assumed that category 4 substances have an effective threshold due to their non-linear dose-effect relationship, they do not appreciably contribute to the human cancer risk if MAK or BAT values are complied with.

The reason why sulphuric acid was classed as a category 4 substance is that tumours developed after exposure to high acid aerosol concentrations. At the same time the MAK value was lowered substantially to 0.1 mg/m3 because under workplace conditions this concentration will not cause any irritation of the respiratory system nor are any relevant changes in the self-cleaning properties of the lung (mucociliary clearance) to be expected.

2,3,7,8-TCDD was assigned to the new category 4 of carcinogenic substances because it has to be assumed that this classical compound with tumour-inducing properties has an effective threshold. As there are no data available which would indicate the exposure threshold leading to TCDD-mediated effects the MAK value was derived from present background exposure. There has been no indication to date that this background exposure causes relevant effects in humans. It was calculated by means of a toxico-kinetic model that the inhalation of 10pg TCDD/m3 per eight-hour day - which is the MAK value - does not significantly increase the background exposure.

Chloroform which the US Environmental Protection Agency (EPA) has also been evaluating for quite some time was also placed in this new category 4. The commission holds the view that cytotoxic effects are responsible for the tumour development demonstrated in animal experiments and that a genotoxic mechanism, i.e. an intracellular interaction with DNA, is irrelevant. The cytotoxic effects can be explained by the fact that chloroform is metabolised and forms reactive metabolites. Extensive animal studies looking into increased cellular growth (cell proliferation) in the target organs liver and kidney as well as a toxico-kinetic model were used as a basis for determining an MAK value of 2 mg/m3 which is not expected to produce any cytotoxic effects and hence does not constitute a cancer risk.

This year no suitable candidates could be found for the new category 5. The criterion for this categorisation is that in spite of the ability of a substance to interact with DNA in the cell the commission does not expect that substance to contribute substantially to the human cancer risk if its MAK value is observed.

MAK values for 16 substances were changed, while for 12 substances the values could be confirmed after in-depth examination. In five cases it was not possible to define MAK values due to a lack of data. It was decided during this review process that the MAK values for fourteen substances should be suspended because the compounds were suspected of being carcinogenic or because the data currently available did not provide an adequate basis for evaluating the health hazard. The intensive reevaluation of older MAK values was continued, also in close co-operation with the European Scientific Committee for Occupational Exposure Limits (SCOEL) and the US Threshold Limit Value (TLV) Commission.

In addition, 24 substances were examined for special prenatal toxicity. In view of new results of animal experiments and the lowered MAK value chloroform was placed in group C which comprises those substances where there is no reason to fear the risk of embryotoxicity if MAK values are observed. Because of its very low workplace limit 2,3,7,8-TCDD was also classed in this category.

This year 41 substances were examined for their potential to sensitise the respiratory tract and the skin. Acrylonitrile, glyoxal, 2,4,6-trinitrophenol, organic mercury compounds as well as other acrylates and woods are new entries in this category. Six substances were marked with the letter H as a warning that apart from inhalation percutaneous absorption may considerably contribute to toxicity at the workplace, e.g. chloroform, cresols and 2,3,7,8-TCDD. This warning was reviewed for another five substances, among them acrylonitrile and methanol.

There are three new entries in the list of BAT values and EKA (biological tolerance values and exposure equivalents of carcinogenic substances). The BAT value for ethylbenzene, for instance, was raised to 1.5 mg/l (blood) or 2,000 mg/g creatinine (urine).

For each of the new entries and changes in the 1999 list of MAK and BAT values the Senate commission provided extensive scientific explanations published by WILEY-VCH, Weinheim. As every year the review and/or new entry of MAK values or the classifications of numerous substances are announced in the "Yellow Pages" of the list of MAK and BAT values.
Information for editors:
A free copy for review may be obtained from the press section of the Deutsche Forschungsgemeinschaft, Kennedyallee 40, D-53175 Bonn, telephone 49-228-885-2210 or -2109, fax 49-228-885-2180.

Deutsche Forschungsgemeinschaft

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