Comparison of the Philosophical Bases of Maximum Allowable Concentrations (MACs) and Threshold Limit Values (TLVs)

Rapid development of chemistry and wide usage of chemical products require specific toxicological studies and hazards evaluation with regard to long-term and combined effects of chemical substances. The setting of standards for chemicals in the working environment is being conducted by occupational hygienists in many countries of the world. Experience on the matter has been accumulated in international and multilateral organizations such as the International Labour Organization, the World Health Organization, the United Nations Environment Programme, the Food and Agriculture Organization and the European Union.

Much has been done in this field by Russian and American scientists. In 1922 studies were launched in Russia to set up standards for chemicals in the air of indoor work areas, and the first maximum allowable concentration (MAC) value for sulphur-containing gas was adopted. By 1930 only 12 MAC values were established, whereas by 1960 their number reached 181.

The American Conference of Governmental Industrial Hygienists (ACGIH) started its work in 1938, and the first threshold limit values (TLVs) list was published in 1946 for 144 substances. The TLVs are to be interpreted and used only by the specialists in this field. If a TLV has been included in the safety standards (the so-called standards of national consensus) and the federal standards, it becomes legal.

At present more than 1,500 MAC values have been adopted for workplace air in Russia. More than 550 TLVs for chemical substances have been recommended in the United States.

Analysis of hygienic standards made in 1980–81 showed that 220 chemicals of the MAC list (Russia) and the TLV list (United States) had the following differences: from two- to fivefold differences were found in 48 substances (22%), 42 substances had five- to ten-fold differences, and 69% substances (31%) had more than ten-fold differences. Ten per cent of the recommended TLVs were 50 times higher than the MAC values for the same substances. The MAC values, in turn, were higher than the TLVs for 16 substances.

The largest divergence of standards occurs in the class of chlorinated hydrocarbons. Analysis of the TLV list adopted in 1989–90 showed a trend toward a reduction of the earlier recommended TLVs compared with the MAC values for chlorinated hydrocarbons and some solvents. Differences among the TLVs and the MACs for the majority of metal aerosols, metalloids, and their compounds were insignificant. The divergences for irritant gases were also slight. The TLVs for lead, manganese and tellurium compared with their MAC analogues disagreed 15, 16 and 10 times, respectively. The differences for acetic aldehyde and formaldehyde were the most extreme—36 and 6 times, respectively. In general, the MAC values adopted in Russia are lower than the TLVs recommended in the United States.

These divergences are explained by the principles used in the development of hygienic standards in the two countries and by the way of these standards are applied to protect workers’ health.

A MAC is a hygienic standard used in Russia to denote a concentration of a harmful substance in the air of the workplace which will not cause, in the course of work for eight hours daily or for any other period of time (but not more than 41 hours per week throughout the working life of an individual), any disease or deviation in the health status as detectable by the available methods of investigation, during the working life or during the subsequent life of the present and next generations. Thus, the concept used in defining the MAC does not allow for any adverse effect on a worker or his or her progeny. The MAC is a safe concentration.

A TLV is the concentration (in air) of a material to which most workers can be exposed daily without adverse effect. These values are established (and revised annually) by the ACGIH and are time-weighted concentrations for a seven- or eight-hour workday and 40-hour workweek. For most materials the value may be exceeded, to a certain extent, provided there are compensatory periods of exposure below the value during the workday (or in some cases the week). For a few materials (mainly those that produce a rapid response) the limit is given as ceiling concentration (i.e., a maximum allowable concentration) that should never be exceeded. The ACGIH states that TLVs should be used as guides in the control of health hazards, and are not fine lines between safe and dangerous concentrations, nor are they a relative index of toxicity.

The TLV definition also contains the principle of inadmissibility of harmful impact. However, it does not cover all of the working population, and it is admitted that a small percentage of workers may manifest health changes or even occupational pathologies. Thus TLVs are not safe for all workers.

According to ILO and WHO experts, these divergences are the result of different scientific approaches to a number of interrelated factors including the definition of an adverse health effect. Therefore, different initial approaches for the control of chemical hazards lead to different methodological principles, essential points of which are presented below.

The main principles of setting hygienic standards for dangerous substances in the air of workplaces in Russia compared with those in the United States are summarized in table 1. Of special importance is the theoretical concept of the threshold, the basic difference between the Russian and the American specialists that underlies their approaches to setting standards. Russia accepts the concept of a threshold for all types of dangerous effects of chemical substances.

Table 1. A comparison of some ideological bases for Russian and American standards

However, the recognition of a threshold for some types of effects  requires  the  distinction  between  injurious  and  non-injurious effects produced by chemical substances. Consequently, the threshold of unhealthy effects established in Russia is the minimal concentration (dose) of a chemical that causes changes beyond the limits of physiological adaptive responses or produces latent (temporarily compensated) pathologies. In addition, various statistical, metabolic, and toxico-kinetic criteria of adverse effects of chemicals are used to differentiate between the processes of physiological adaptation and pathological compensation. Pathomorphological changes and narcotic symptoms of earliest impairment have been suggested in the United States for the identification of injurious and non-injurious effects. It means that more sensitive methods have been chosen for the toxicity evaluation in Russia than those in the United States. This, therefore, explains the generally lower levels of MACs compared to TLVs. When the detection criteria for injurious and non-injurious effects of chemicals are close or practically coincide, as in the case of irritant gases, the differences in standards are not so significant.

The evolution of toxicology has put into practice new methods for the identification of minor changes in tissues. These are enzyme induction in the smooth endoplastic reticular hepatic tissue and reversible hypertrophy of the liver. These changes may appear after exposure to low concentrations of many chemical substances. Some researchers consider these to be adaptive reactions, while others interpret them as early impairments. Today, one of the most difficult tasks of toxicology is obtaining data that show whether enzyme disturbances, nervous system disorders and changes in behavioural responses are the result of deteriorated physiological functions. This would make it possible to predict more serious and/or irreversible impairments in case of long-term exposure to dangerous substances.

Special emphasis is placed on the differences in the sensitivity of methods used for the establishment of MACs and TLVs. Very sensitive methods of conditioned reflexes applied to studies of the nervous system in Russia have been found to be the main cause of divergences between the MACs and the TLVs. However, the use of this method in the process of hygienic standardization is not obligatory. Numerous methods of different sensitivities are normally used for the developing of a hygienic standard.

A great number of studies conducted in the United States in connection with the setting-up of exposure limits are aimed at examining the transformation of industrial compounds in the human body (routes of exposure, circulation, metabolism, removal, etc.). Methods of chemical analysis used to establish the values of TLVs and MACs also cause divergences due to their different selectivities, accuracies and sensitivities. An important element usually taken into consideration by OSHA in the standardization process in the United States is the “technical attainability” of a standard by industry. As a result, some standards are recommended on a basis of the lowest presently existing concentrations.

MAC values in Russia are established on a basis of the prevalence of medico-biological characteristics, whereas the technological attainability of a standard is practically ignored. This partly explains lower MAC values for some chemical substances.

In Russia MAC values are assessed in toxicological studies before a substance is authorized for industrial use. A tentative safe exposure level is established during the laboratory synthesis of a chemical. The MAC value is established after animal experiments, at the design stage of the industrial process. The correction of the MAC value is carried out after evaluation of working conditions and workers’ health when the substance is used in industry. Most of the safe levels of exposure in Russia have been recommended after experiments on animals.

In the United States a final standard is established after a chemical substance has been introduced in industry, because the values of permissible levels of exposure are based on the assessment of health. As long as the differences of principle between the MACs and the TLVs remain, it is unlikely to expect the convergence of these standards in the near future. However, there is a trend towards the reduction of some TLVs that makes this not so impossible as it may seem.

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