Document Zn3YwLnqZNXbzZz1e99OeprEZ

ELEMENTS OP INDUSTRIAL TOXICOLOGY TABLE 40-B. THRESHOLD LIMIT VALUES FOR 1983 (Continued) Appendix B THRESHOLD LIMIT VALUES FOR MIXTURES When two or more hazardous sub stances are present, their combined effect, rather than that of either individually, should be given primary consideration. In the absence of information to the con trary. the effects of the different hazards should be considered as additive. That is, if the sum of the following fractions, Ci C> Cp T, + T, + T. exceeds unity, then the threshold limit of the mixture should be considered as being exceeded C indicates the observed atmos pheric concentration, and T the corres ponding threshold limit, (See Example 1A a ). Exceptions to the above rule may be made when there is good reason to be hove that the chief effects of the different harmful substances are not m fact addi tive, but independent as when purely lo cal effects on different organs of the body are produced bv the various components of the mixture In such cases the threshold limit ordinarily is exceeded only when at least one member of the senes itself has a value exceeding unity, (See Example lAb ). Antagonistic action or potentiation may occur with some combinations of atmos pheric contaminants Such cases at pres ent must be determined individually Po tentiating or antagonistic agents are not necessarily harmful by themselves Po tentiating effects of exposure to such agents bv routes other than that of inhala tion is also possible, e.g. imbibed alcohol and inhaled narcotic (tnchloretbylene) Potentiation is characteristically exhibited at high concentrations, less probably at low When a given operation or process characteristically emits a number of harm ful dusts, fumes, vapors or gases, it will freauently be only feasible to attempt to evaluate the hazard by measurement of a single substance In such cases, the threshold limit used for this substance should be reduced by a suitable factor, the magnitude of which will depend on the number, toncity and relative quan tity of the other contaminants ordinarily present Examples of processes which are typi cally associated with two or more harmful atmospheric contaminants are welding, automobile repair, blasting, painting, lac quering, certam foundry operations, die sel exhausts, etc. (Example 2 ) EXAMPLES 1A General case, where air is analyzed for each component a ADDITIVE EFFECTS Air contains 5 ppm of carbon tetrachloride (TLV, 10), 20 ppm of ethylene dichlonde (TLV, 50) and 10 ppm of ethylene dibronude, (TLV, 25). _5 10 _20_ + 50 + 10 25 _ - 65 50 _ ~ 13 Threshold limit is exceeded. b. INDEPENDENT EFFECTS Air contains 015 mg/cu m of lead (TLV, 02) and 0 7 mg/cu m of sulfunc acid (TLV, 1). 0X5 020 = 0.75, 0.7 = 07 Threshold limit is not exceeded IB Special case when source of contami nant is a mixture and atmospheric composition is assumed similar to that of original material, te vapor pressure of each component is the same at the observed temperature a ADDITIVE EFFECTS, approxi mate solution 1 A mixture of equal parts (1) trichloroethylene (TLV, 100),' and (2) methyl chloroform (TLV. 350). C, _Ct_ _ C,, 100 + 350 ~ T* Cl -- Cl =-|- Cm Ci . Ci _ 2Ci 100 T 350 - T,, 7C, 2Cj_ _ 2Ci 700 + 700 ~ T,, 40-9