The addition of adulterants is called adulteration. Use of toxic adulterants such as levamisole has been documented. Cooking may not eliminate adulterants, but may reduce microorganisms. It provides sample identification and mixture analysis, allowing the detection of impurities and adulterants. Some adulterants increase emissions of harmful pollutants from vehicles, worsening urban air pollution. Historically, the use of adulterants has been common; sometimes dangerous substances have been used. In wartime adulterants have been added to make foodstuffs "go further" and prevent shortages. As listed above, intentionally added heavy metals and drug adulterants are found in many products. These kits claim to be able to identify common adulterants in ecstasy. With some drugs, adulterants are sometimes added to make the product more appealing. The assumption that tar has less adulterants and diluents is a misconception. Its speciality is the use of cows' milk without any adulterants. For this reason it is essential to use multiple different tests to show all adulterants. Also, adulterants may be added in order to increase weight or modify appearance.