How much second hand cannabis smoke will test positive in hair strand drug test ?

Debbie,

You have ask an interesting question. There isn't a quantitative answer (i.e. how much) but it can be stated that there is a very real chance that hair drug tests are influenced by contamination from second hand and side stream smoke. (5)

A factor complicating a response is that we don't know how cannabis metabolites (THC, THC-A, THC-COOH, CBN, CBD, etc) get into the hair in the first place. But external exposure is certainly one way. (6)

The idea that cannabis is incorporated into hair via circulation is very questionable. Some of the metabolites might enter via circulation but very little. Perhaps a very minute amount COOH enters via circulation but none of THC which seems to enter via exposure to the external hair shaft probably by way of the sebaceous glands. (1,2,)

Hair has proved to be a very poor matrix for cannabis testing. Infrequent and even moderate use is seldom detected via hair testing. It is effective for detecting long-term heavy use but little else. (1,4)

Your question also necessitates consideration of laboratory procedures as hair testing requires a pre-wash to remove external contaminants. Because all drugs must be extracted from hair (not actually testing the hair) any external contamination not removed by the chemical wash becomes part of the solution tested. THC metabolites can be found in hair even following multiple washings.(2)

There is no standardized formula for chemical wash. Different labs use different washes. Overall the wash process is only about 85% effective. Add to this that tests for cannabis are conducted in the range of picograms (parts per trillion), it becomes apparent that contamination is a real problem. External contamination by cannabis smoke can and does cause false positives. (5)

1) Huestis (2007)
2) Auwater (2010)
3) Nakahara (1999)
4) Galanter & Kleber, 2011, p. 58
5) Thorspecken (2004)
6) Vogliardi (2008) Doctoral thesis.