Sometimes when a hair test comes back positive, people say things like, “I was just around someone using it” or “I was at a party where people were doing it.” At Cansford, we recognise that traces of drugs can occasionally settle on hair from the environment. This is called external contamination. While external contamination can happen, it’s very rarely enough to significantly affect a hair test result.
What is external contamination?
External contamination happens when traces of drugs settle on the hair surface, for example through close contact with users or exposure to smoke. This can include sweat, sebum, or environmental contact with substances. These traces don’t pass through the body, so they don’t create metabolites — the chemical markers of actual ingestion.
Metabolites: the key to confirming use
Metabolites are the chemical by-products produced when the body processes a substance. They only end up in the hair if the drug or alcohol has entered the body. Once embedded in the hair, these metabolites stay in place, creating a chemical record of substance use over weeks or months.
Each parent drug can produce multiple metabolites depending on the form in which the parent drug was used, and in some cases also how it was taken. Testing for both the parent drug and its metabolites allows us to reliably confirm whether the substance was actively ingested.
Separating exposure from use
All hair samples are washed during preparation to remove any external drug deposits. The wash liquid can be analysed separately if needed. Sometimes, trace levels of a substance can appear in both the hair and the wash. This doesn’t automatically indicate active use or contamination — it is the ratio between the two helps us determine the likely source.
By looking at both the hair and the wash results together, we can distinguish between external contact and actual ingestion. We can also look at patterns along the hair strand to see whether drug levels are consistent with ongoing use or environmental exposure, which gives a much clearer picture of what’s really happening. This ensures that positive results reflect substance use rather than exposure.
What this means for interpreting results
External contamination can happen, but with our methods, it’s usually clear whether it’s a plausible explanation for a positive result. The combination of the way we conduct hair strand analysis, with the added context of wash testing, allows us to interpret results with confidence.
Being around drugs doesn’t automatically mean a positive result. At Cansford, we can separate environmental exposure from active use, giving a reliable, accurate picture of substance intake over time.
