False positives are when a drug test shows a positive result for a substance that you did not use. This can happen due to various reasons, such as cross-reactivity with other substances, laboratory errors, or contamination. One possible cause of a false positive for weed is exposure to smoke from marijuana from other people. This can happen if you are in a closed or poorly ventilated space where someone is smoking weed, or if you share a smoking device with someone who uses weed. However, the risk of getting a false positive for this cause is very low, as the amount of THC that you can inhale from secondhand smoke is minimal and unlikely to reach the detection thresholds of most drug tests. A 2015 study found that non-smokers who were exposed to extreme smoke from marijuana in a chamber for 15 minutes had detectable levels of THC in their urine, but only for a few hours and below the cutoff levels for most drug tests. To avoid getting a false positive from this cause, you should avoid being around people who are smoking weed, especially in enclosed or poorly ventilated spaces. You should also avoid sharing any smoking devices with weed users, such as pipes, bongs, or vaporizers.