Environmental pollution is associated with adverse health effects experienced or measured in the short or long term, that clinically may be described as either acute or chronic. Usually acute effects follow sudden and severe exposure and rapid absorption of the substance. On the contrary, chronic effects may not be immediately observed and follow prolonged or repeated exposures over more or less relatively long periods. Examples are carbon monoxide poisoning, and lead poisoning or cancer, respectively. However, clinically acute effects may also be observed after long-term exposures (e.g., convulsions following long-term exposures to lead) and chronic after-effects following short-term exposures, as is the case with nitrogen dioxide. Owing to differences in vulnerability, in any population there will be a range of responses to exposure to a particular agent, with the susceptible or vulnerable persons occupying the extremity of the distribution corresponding to a lower exposure than that of the majority of the population. Vulnerability may be based on genetically determined factors or physiological characteristics such as age (children and the very elderly are usually more vulnerable), pregnancy, or general health status, among others.