Fig. 8.1 Climatechange,pastandfuture.(A)CorrelationofCO2levelsmeasuredattheMaunaLoaObservatoryinHawaiiwithaverageglobaltemperaturetrendssincethemid-1960s.“Globaltemperature”inanygivenyearwasdeducedattheHadleyCenter(UnitedKingdom)frommeasurementstakenatmorethan3000weatherstationslocatedaroundtheglobe.
(B)Predictedtemperatureincreasesduringthe21stcentury.Differentcomputermodelsplotanticipatedrisesinglobaltemperaturesof2°Cto5°Cbytheyear2100.(A, Courtesy of Dr. Richard Aster, Department of Geosciences, Colorado State University, Fort Collins, Colorado.) temperatures may already be near or above crop tolerance levels; it is estimated that by 2080, agricultural productivity may decline by 10% to 25% in some developing countries as a consequence of climate change.
Beyond these disease-specific effects, it is estimated that the melting of glacial ice, particularly in Greenland and other parts of the Northern Hemisphere, combined with the thermal expansion of warming oceans, will raise sea levels by 2 to 6 feet by 2100. Approximately 10% of the world’s population—roughly 600 million people—live in low-lying areas that are at risk for flooding even if the rise in ocean levels is at the low end of these estimates. For example, a rise in sea level by 1.5 feet will submerge 70% of the land mass of Maldive islands by 2100 and a 3-foot rise will inundate 100% all of the islands by 2085. The resulting displacement of people will disrupt lives and commerce, creating conditions ripe for political http://ebooksmedicine.net unrest, war, and poverty, the “vectors” of malnutrition, sickness, and death. Worldwide recognition of the catastrophic effects of climate change led in late 2015 to a  historic meeting of 196 countries in Paris, France, at  which the participating countries agreed to the following objective:
Holding the increase in the global average temperature to well below 2°C above preindustrial levels and to pursue efforts to limit the temperature increase to 1.5°C above preindustrial levels, recognizing that this would significantly reduce the risks and impacts of climate change.
Toxicology is defined as the science of poisons. It studies the distribution, effects, and mechanisms of action of toxic agents. More broadly, it also includes the study of the effects of physical agents such as radiation and heat. Approximately 4 billion pounds of toxic chemicals, including 72 million pounds of known carcinogens, are produced each year in the United States. In general, however, little is known about the potential health effects of chemicals. Of the approximately 100,000 chemicals in use in the United States, less than 1% have been tested experimentally for health effects.
We now consider some basic principles regarding the toxicity of exogenous chemicals and drugs.
The definition of a poison is not straightforward. It is a quantitative concept strictly dependent on dosage. The quote from Paracelsus in the 16th century that “all substances are poisons; the right dosage differentiates a poison from a remedy” is perhaps even more valid today, in view of the proliferation of therapeutic drugs with potentially harmful effects.
Xenobiotics are exogenous chemicals in the environment that may be absorbed by the body through inhalation, ingestion, or skin contact (
Fig. 8.2
Chemicals may be excreted in urine or feces or eliminated in expired air, or they may accumulate in bone, fat, brain, or other tissues.
Chemicals may act at the site of entry, or they may be transported to other sites. Some agents are not modified on entry in the body, but most solvents and drugs are metabolized to form water-soluble products (detoxification) or are activated to form toxic metabolites.