Hair analysis may refer to the chemical analysis of a hair sample, but can also refer to microscopic analysis or comparison. Chemical hair analysis may be considered for retrospective purposes when blood and urine are no longer expected to contain a particular contaminant, typically three months or less. Its most widely accepted use is in the fields of forensic toxicology and, increasingly, environmental toxicology. Several alternative medicine fields also use various hair analyses for environmental toxicology, but these uses are controversial, evolving, and not standardized. Microscopic hair analysis has traditionally been used in forensics as well. Analysts examine a number of different characteristics of hairs under a microscope, usually comparing hair taken from a crime scene and hair taken from a suspect. It is still acknowledged as a useful technique for confirming that hairs do not match. But, DNA testing of evidence has overturned many convictions that relied on hair analysis.