I smoked a half a joint 2 and a half months ago.Will I be able to pass a hair drug test. I do exercise a lot and drink a lot if water.

The ways that drugs are incorporated into hair and the nature of hair drug testing negate the value of dilution and exercise once the drug has been deposited in the hair.

However, drugs are deposited (incorporated) into hair at very different rates and in different ways.

Cannabis metabolites have the lowest rate of incorporation of all drugs therefore are not detected simply because the metabolite THC-COOH* 1) doesn't bind to hair components and/or 2) doesn't bind in sufficient quantity to rise above the level of detection on testing.

The result is that low level cannabis use is not discovered in HDT.

*this is the only metabolite that can be used to show direct ingestion.

Ref:
Nakahara, Y (1999) Hair analysis for abused and therapeutic drugs. Journal of Chromatography B, 733 (1999) 161–180, [p 174 contains graph of incorporation by drug type]

Musshoff F, Madea B. (2006) Review of biologic matrices (urine, blood, hair) as indicators of recent or ongoing cannabis use. There Drug Monit. Apr 2006;28(2):155-63

DuPont 2008, Random Student Drug Tests Are They Effective for Identifying Occasional Drug Users? p. 8

Marilyn A. Huestis (2007) Human Cannabinoid Pharmacokinetics. Chem Biodivers. 2007 August; 4(8): 1770–1804. Sec 3.5