Toxic chemicals synergistic effects

SYNERGISTIC When the combined effect, e.g. of exposure to a mixture of toxic chemicals, is greater than the sum of the individual effects. 

In reality, medical scientists cannot yet predict the toxic effects to individual humans from consuming low-level chemical mixtures that permeate our food. Although scientists know virtually nothing about the toxic effects of chemical mixtures, they do know that there can be synergistic effects from multiple chemical exposures.1... 

As seen further on in this chapter, individual PEEP index values express a condensed portrait of an effluent s hazard potential which takes into account several important ecotoxicological notions (toxic intensity and scope in terms of biotic levels impacted, bioavailability, persistence of toxicity and effluent flow). Unlike wastewater investigations limited to chemical characterization, this bioassay-based scale reflects the integrated responses of several representative toxicity tests to all interaction phenomena (antagonistic, additive and/or synergistic effects) that can be present in effluent samples. 

Many of the factors that can affect organisms adversely are inherent stressors, including availability of nutrition, water quality, temperature and other climatic extremes, disease, and predation. It is important to be able to separate effects of inherent stressors from those of toxic chemicals. There are often important synergistic relationships between inherent stressors and the effects of toxic chemicals. Organisms that are under stress from inherent stressors are likely to be more susceptible to the effects of xenobiotic toxicants. 

Another explanation to the observed toxicity in some of the paints could be that the included chemicals exhibit synergistic effects and thereby together are more dangerous than the additive effect of the single chemicals. Impurities of the included chemicals could also be part of an explanation. 

A synergistic effect occurs when the combined toxic effect of two chemical substances is much greater or worse than the sum of the effects of each by itself. Synergism is similar to adding 2 + 2 and getting 5 as the result. 

Exposure to a single PAH compound seldom occurs in situations of relevance to local law enforcement agencies or defense forces. Often, people are exposed to PAHs in combination with other toxicants such as aliphatic hydrocarbons and/or metals (NRC, 2005). Information on toxicity of chemical mixtures, of which PAHs are a constituent, is lacking. Until such studies are undertaken in animal models, it is difficult to establish whether PAHs have an additive or a synergistic effect in determining neurotoxicity. 

The human cost is vividly narrated in the various books mentioned in Section II of this chapter. While there is no doubt that the major toxic chemical responsible for acute and chronic toxicity in Bhopal was MIC, there is also ample evidence that the toxic fimies contained other noxious substances. Whether or not these additional chemicals exerted additive or synergistic effects on MIC toxicity cannot be stated categorically. It is reasonable to assume that major effects, both acute and chronic, were due to MIC. 

The data in Table 3.3 are remarkable in that despite the fact that the chemical and toxicological properties of the compounds studied vary widely, the effects of these chemicals on the toxicity of 2,4-D are predicted by Kqw values. The authors hypothesized that combinations of lipophilic and hydrophilic compounds would show synergistic effects resulting from membrane damage by lipophilic species and increased uptake of hydrophilic species. 

Here, again, numerous mixtures are possible. An example of a toxic binary mixture of chemicals on this list shows a synergistic effect. When administered together, ethanol inhibits the metabolism of ethyl acetate, resulting in greater toxicity of ethyl acetate. I29l... 

The effects of individual substances on a wide range of species in the environment are rarely understood and, depending on the adequacy and the quality of the information, an uncertainty factor usually is applied. There are a number of limitations to this approach. It is substance specific and of the total number of listed chemicals less than 1% have sufficient information to derive a safe level. Uncertainty factors can move towards overly stringent control, the approach is unable to consider toxic effects of complex mixtures (which is the usual route of entry) and it takes no account of additive or synergistic effects. 

Data concerning the toxicity of the four discussed toxic minerals are presented in Tables 4.5 and 4.6. The uptake of elements is not entirely independent of one another. Elements of similar chemical properties tend to be taken up together. Sometimes one element has an inhibiting effect on another, or there can be a synergistic effect, e.g., enhancement of absorption of calcium in the presence of adequate amounts of phosphorus, or cadmium and lead hindering calcium and iron absorption, or zinc and copper antagonism and their influence on the ratio of Zn/Cu on copper deficiency. 

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