Routes of entry for toxic substances

No matter which mode of entry a toxin takes, the health effects can be either local or systemic, depending upon the physical and chemical characteristics of the substance. Local effects are the tissue reactions of the body areas that come in direct contact with the contaminant. Systemic effects, on the other hand, occur in the target sites of the body, which are not the initial contact locations, but which, because of their affinity for that particular substance, readily absorb it. This reaction may occur far away from the point of initial contact. If an employee were working with sulfuric acid and inhaled sufficient quantities of it, irritation of the upper respiratory system would be likely (irritation of the nose, bronchi, trachea, etc.). Inhalation of heavy metals, such as lead, would have systemic effects (lead does not have a substantial impact upon the respiratory system, which in this case is the site of contact). However, the primary dangers from lead are to the central nervous system, the blood, and the kidneys. 

When two or more substances are taken into the body, there are three possible interactions that may take place  

From toxicological studies, we know and can predict the amount of injury caused by many individual substances. When we combine these substances, and the effects are as predicted, we call this additive. We express this as 1 + 1 = 2. When we combine two or more substances and a lower overall effect is observed, that is, their combined effect is less than the sum of their individual effects, we call this antagonism. This is expressed as 1 + 1 2. When we combine two or more substances and the overall effect is greater than the predicted effects, that is, their combined effect is greater than the sum of their individual effects, we call this synergism. This may be expressed as 1 + 1 2. 

There are numerous ways to classify toxic materials. In general, toxic materials are categorized according to their physical or chemical characteristics or the physiological organs that they target. The fol- 

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