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However, experts said that in practice, hair differences are just one of many factors between individuals or ethnic groups that can influence the results of a drug test.
Many major police departments, as well as employers nationwide, use hair samples to test for illegal drugs. Proponents of the tests say they are easier to take and more secure than urine tests, which are prone to tampering.
Studies in animals and people, however, have shown that both the colour and consistency of hair may affect the concentration of chemicals that accumulate in it.
In the latest experiment, researchers at the University of Utah in the US studied the affect of hair colour on drug deposition in 31 people with red, blond, brown, or black hair.
Read: The easy way for employers to test if their workers are taking drugs
Black hair retains more chemicals
Each day for five days, the subjects took oral doses of ofloxacin, a potent antibiotic, and the painkiller codeine. The Utah scientists then cut hair samples from the participants four to seven weeks after the drugs were given, and tested them with a fluorescent screening technique.
Redheads accumulated the least amount of ofloxacin in their hair, about 1.5 nanograms per milligram. That was about 15 times less than those with black hair, the researchers say. Blondes and people with brown hair amassed 3.5 and 6 nanograms per milligram, respectively. The results were similar for codeine.
Matt Slawson, a research toxicologist at the University of Utah and a co-author of the study, says the hair differences appear to be due to higher or lower concentrations of the pigment molecule melanin. Some drugs show an affinity for melanin, which is present in hair and other cells in the body. And darker hair contains a variety of melanin that is particularly attractive to chemicals.
Slawson says the significance of his group's finding is open to debate. If companies have a zero-tolerance policy for drug tests, then hair-splitting over melanin is irrelevant.
If, however, employees aren't punished for having barely detectable traces of drugs in their hair - the possible result of inadvertent contact, such as walking into a room where others are smoking a joint - then people with dark hair might be at a substantial disadvantage.
Read: Are hair drug tests accurate?
Bill Thistle, general counsel for the employee drug-testing firm Psychmedics, disputes the Utah results, saying that other studies have failed to find a hair colour bias. Only one comparison matters to employers: who's doing drugs and who isn't.
Employees who have brought the hair bias issue to court have uniformly lost their pleas, he says, on the grounds that a positive test - whether weak or strong - is still a positive test.
Steven Magura, director of treatment research at the United States Developmental and Research Institute, says a hair bias isn't likely to be important when comparing testing outcomes between racial or ethnic groups, or even individuals. A multitude of factors, from body weight to genetics, affect the way a person processes drugs.
On the other hand, he says, a person with dense, black hair, which stores up drugs more readily than fine, blonde hair, might test positive long after he had actually used the illegal substance, whereas a blonde might test negative sooner.
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