Here is the FAQ's from the company that is basically the 800 pound gorilla of the hair testing industry.

http://www.psychemedics.com/faq/

This is a key point here:

Is there a risk that the results of a hair test can be affected by environmental contamination?

Psychemedics utilizes several independent approaches which, in various combinations, rule out the possibility of a positive result from external contamination. The first method involves extensive chemical washing of the hair specimen prior to screening, followed by analysis of the content of the wash. This wash analysis is a critical step to ensure that any contamination is effectively accounted for. Additionally, Psychemedics measures the presence of metabolites. If drugs were in the air or on a person’s hands and thereby got on a person’s hair from outside, the drug would be present as the drug substance itself, and not as certain metabolites or with metabolite/parent drug ratios which are known to be produced by ingestion. Psychemedics’ ability to distinguish and measure metabolites with its highly sensitive GC/MS/MS or LC/MS/MS equipment is one criteria used to eliminate the possibility of false positives from external contamination. In addition, any positive internal contamination (e.g., from passive inhalation or even poppy seed consumption) is distinguished from deliberate drug use by setting GC/MS/MS or LC/MS/MS cut-off levels above those which can be produced by passive internal exposure. Studies have shown that the combination of extensive washing, metabolite analysis, and proper cut-off levels are necessary to avoid false positives due to external contamination.