Occupational Health and Safety Act, 1993 (Act No. 85 of 1993)

Regulations

Regulations for Hazardous Chemical Agents, 2020

Annexures

Annexure 3 : Hazardous Chemical Agent Guidelines

Guidance on medical surveillance and biological monitoring

Mixed exposures

Assessment and control

Purchase

81.

With all types of mixed exposures it is essential that assessments be based on the concentrations of each of the constituents in air to which workers are exposed. Depending on the nature of the constituents and the circumstances of use, the relative concentrations of the constituents in air may differ considerably from those in the liquid or solid source material. The composition of the bulk material should not be relied on for assessment unless there is good evidence for doing so.

82.

The ways in which the constituent agents of a mixed exposure interact vary considerably. Some mixed exposures involve agents that act on different body tissues or organs, or by different toxicological mechanisms, these various effects being independent of each other. Other mixtures will include agents that act on the same organs, or by similar mechanisms, so that the effects reinforce each other and the agents are additive in their effect. In some cases the overall effect is considerably greater than the sum of the individual effects and the system is synergistic. This may arise from mutual enhancement of the effects of the constituents or because one agent potentiates another, causing it to act in a way which it would not do alone. [Query: this is a duplication of paragraph 80, should it be removed or replaced?]

(a)

Synergistic agents: known cases of synergism and potentiation are considerably less common than the other types of behaviour in mixed exposures. However, they are the most serious in their effects and require the strictest control. They are also the most difficult to assess and wherever there is reason to suspect such interaction, specialist advice should be obtained;

(b)	Additive agents: where there is reason to believe that the effects of the constituents are additive, and where the exposure limits are based on the same health effects, the mixed exposure should be assessed by means of the formula-

R280 Annex 3 (82) formula (1)

Here Em is the exposure for the mixture, and C1, C2, etc. are the time-weighted average (TWA) concentrations of constituents in air.

OEL1, OEL2, etc. are the corresponding exposure limits. The use of this formula is only applicable where the additive agents have

been assigned OELs which relate to the same reference period in the list of promulgated OELs. If the equation generates a result that

is > 1, then the exposure limit for the mixture (Em) has been exceeded. If one of the constituents has been assigned an OEL-ML, then

the additive effect should be taken into account in deciding the extent to which it is reasonably practicable to further reduce

exposure; and

(c)	Independent agent: where no synergistic or additive effects are known or considered likely, the constituents can be regarded as acting independently. It is then sufficient to ensure compliance with each of the OELs individually.

83.

The above steps provide basic protocol for assessment of mixed exposures. It is open to persons responsible for control of exposure to treat all nonsynergistic systems as though they were additive. This avoids the need to distinguish additive and independent systems and can be regarded as the most prudent course, particularly where the toxicity data are scarce or difficult to assess.