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Contamination is the presence of a minor constituent in another chemical or mixture, often at the trace level. In chemistry, the term usually describes a single chemical, but in specialized fields the term can also mean chemical mixtures, even up to the level of cellular materials.

All chemicals contain some level of contamination. Contamination may be recognized or not and may become an issue if the contaminated chemical is mixed with other chemicals or mixtures and causes additional chemical reactions. The additional chemical reactions can sometimes be beneficial, in which case the label ‘contaminant’ is replaced with reactant or catalyst. If the additional reactions are detrimental, other terms are often applied such as toxin, poison or pollutant depending on the chemistry involved. A major fraction of chemistry is involved with identifying, isolating, and studying contaminants.

"Contamination" also has more specific meanings in some sciences:

• In environmental chemistry the term is in some cases is virtually equivalent to pollution, where the main interest is the harm done on a large scale to humans or to organisms or environments that are important to humans.
• In food chemistry and medicinal chemistry, the term "contamination" is used to describe harmful intrusions, such as the presence of toxins or pathogens in food or medicinal drugs.
• In forensic science, a contaminant can be complex materials such as hair or skin particles arising from sources not related to the ongoing investigation.
• In the biological sciences accidental introduction of "foreign" material ('contaminantion') can seriously distort the results of experiments where small samples are used. In cases where the contaminant is a living microorganism, it can often multiply and take over the experiment, especially cultures, and render them useless.
• In geology and especially geochemistry, it can have similar effects where even a few grains of "modern" dust can distort results of sophisticated experiments.
• the term "contamination" is sometimes used to describe accidental transfers of organisms from one natural environment to another.
• it has even been used in the "Directed panspermia" hypothesis about the origin of life on Earth, which suggests that visiting aliens accidentally infected the planet with microbes from their own world.^[1]

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