Toxic compounds and Toxicology

Toxics are the chemical and physical agents that have adverse effects on living organisms, and toxicology is the science dealing with toxic agents. The word toxic may be considered to be synonymous with harmful in regard to the effects of chemicals [17]. 

Dose is the number one factor in toxic effect determinations. From statistically treated dose-response data, the dose (in mg/kg body weight) killing 50% of the sample population is designated as the median lethal dose (MLD or LD50). However, one should keep in mind that, LD50 values may not accurately reflect the full spectrum of toxicity or hazard all the time, because some chemicals with low acute toxicity may have carcinogenic (or endocrine) effects even at very low doses that produce no evidence of acute toxicity at all. 

Some toxic agents can also be absorbed by the skin. Since skin is permeable, toxic gases can be absorbed and can be distributed by the blood stream quickly through skin penetration. Cuts and other abrasions can accelerate the absorption process. 

If a chemical is completely excreted, then succeeding doses have no increased effect, hut if a residue remains, then it is possible for the second dose to add to the first and, if doses are repeated often enough, to reach a level high enough to be toxic. In this context, it is obvious that, the water solubility - tissue reactivity and blood to gas phase partition coefficient values of the toxicants are all important in cases of exposure to gases indoors. 

It is also worth noting the differences between acute toxicity (effects that occur shortly after a single exposure) and chronic toxicity (delayed effects that occur after longterm, repeated exposures). 

---