Situational chemical exposure

Independent of combustibility or reactivity, chemicals may exhibit hazardous properties that affect fire protection, in particular firefighting. Chemicals may be inherently toxic or radioactive. In either situation, potential exposure of smoke/gases to personnel from the burning of materials exhibiting these characteristics needs to be addressed. 

Antagonistic effect The effect of a chemical in counteracting the effect of another for example, the situation where exposure to two chemicals together has less effect than the simple sum of their independent effects such chemicals are said to show antagonism. 

Of course in some situations of exposure to chemicals, such as around waste disposal areas or chemical factories, exposure is to a mixture of possibly many different chemicals. These may interact in a variety of ways (e.g., additivity, synergism, antagonism, potentiation, see above). 

Occupational exposure to chemical substances almost invariably involves multiple chemicals. That situation may result in PK interactions, which may affect the relationship between the atmospheric concentration of the parent chemical and the associated biomarker concentration (Viau 2002). For example, such an interaction is known to occur between ethylbenzene and the xylene isomers (Jang et al. 2001). Commercial xylene contains about 20% ethylbenzene, which modifies the slope of the relationship between urinary methylhippuric acid (MHA) and airborne xylene concentrations. That kind of interaction is unlikely at the subparts-per-million exposure concentrations seen in the general population. But because the BEI for MHA was obtained from the relationship observed after exposure to commercial xylene, thereby taking the interaction into account, the slope of the relationship cannot be extrapolated to the subparts-per-million range. Similar PK interactions have been observed for other mixtures but only at concentrations nearing or exceeding the occupational exposure limits (Viau 2002), so it would be a priori reasonable to consider extrapolation of the relationship between biomarker concentrations and those of their parent chemicals. For example, Tardif et al. (1991) demonstrated that, provided inhalation exposure to a mixture of toluene and xylene was kept below their airborne occupational exposure limits, there were no PK interactions between the compounds that affected the linear relationship between airborne parent-chemical exposure and urinary-metabolite concentrations. However, such an interaction was apparent at higher concentrations. 

When you do find out through the doctors practicing environmental medicine what is wrong with your child (or yourself) and you need to avoid all chemical exposures (including chemically perfumed products), in most cases your family and friends are unwilling to comply — so you re in a lose-lose situation. 

Many of the studies on the effects of chemical exposures on the growth and development of the lungs have been performed in experimental animals however, patterns of lung development differ between animals and humans. Because of these differences, extreme care must be taken when extrapolating the results from animal studies to human situations. 

Synergy applies to a situation involving exposure to two or more toxic chemicals. If this occurs the result is that the combination of chemicals is much more toxic than expected (that is, it is greater than the sum of the parts). In contrast, additivity is simply the sum of the two effects of the individual chemicals, that is the expected effect. 

However, in such situations, a more scientifically accurate approach is to use methods such as risk based on surrogates or threshold of toxicological concern (TTC) to estimate the risk for chemicals lacking sufficient data, rather than modifying the approach to risk estimation for individual chemicals to account for otho-chemical exposures [e.g., see reviews by Kroes et al. (2005) and Dolan et al. (2005)]. 

Patients with MCS believe that their symptoms result from low-level chemical exposure. Many patients respond to signs by avoiding situations of per-... 

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