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How much should I worry about secondhand cannabis smoke?

  • Author: Scott Woodham
  • Updated: September 28, 2016
  • Published February 3, 2015

For the inaugural installment of Highly Informed, our new regular feature that seeks to answer your questions about legal cannabis in Alaska, Melanie asks a few related questions about second-hand smoke.

Readers should keep in mind an important context right up front. There are many ways to consume marijuana, and burning it is just one of them. The answers below pertain to smoking, not vaporizing (which involves no combustion) or eating an infused product.

What happens if I walk down the street and walk through someone's smoke, and then I have a reaction and heaven forbid hurt someone? Who is responsible?

Walking through a cloud of marijuana smoke outside is extremely unlikely to cause "a reaction" in terms of getting you high. Science has found that a cannabis smoker's lungs absorb essentially all of the psychoactive compounds present at inhalation, so there's basically none being exhaled into the atmosphere.

Since you won't be in danger of catching an unwanted buzz, you'd be responsible for any harm you do to another person. If by "reaction" you mean something more like road-rage than intoxication, then it's also still your responsibility.

Can I fail a drug test from walking through someone's second-hand smoke?

That is very unlikely. The threshold for a positive drug test is customarily set high enough, and the THC content of an outdoor cloud is low enough and dispersed enough, that you're in very little danger of failing a drug test that way. Studies have shown that non-smokers do show detectable levels of marijuana in their bloodstream and urine after second-hand exposure, but most studies have focused on enclosed areas. According to most, however, resulting levels in non-smoking test subjects were not large enough to trigger a positive drug test after a day, and in many cases mere hours. In 2010, researchers concluded that passively inhaling second-hand smoke in a real-life scenario was unlikely to trigger a positive drug test in someone who didn't directly smoke any herb. A more recent study published in the Journal of Analytical Toxicology last October found similar results.

However, it's worth noting that no one should take any chances. Sitting a few hours in a smoke-filled room or car on the way to the testing facility is a bad idea. But just walking down the street? No worries.

Second-hand smoke from cigarettes is bad. How bad is second hand smoke from marijuana?

The short answer is the jury's still out on that, and depends on how we define "bad." Any amount of smoke could pose a danger to people with certain health problems. It's also worth noting that research on second-hand marijuana smoke nearly always involves rolling marijuana into cigarette papers, which is only one of the ways marijuana is smoked in real life. Pipes and bongs don't usually involve burning paper, and usually do not give off continuous smoke the way a lit cigarette does.

Both tobacco and marijuana smoke contain chemicals that are known to cause a variety of health problems in humans. A study in 2008 that used machines to "smoke" cannabis cigarettes found that marijuana "mainstream" smoke (the kind coming out of the business end of a joint) showed a lower concentration of some toxic chemicals than in tobacco smoke, but more in the "sidestream" smoke (the smoke coming from the burning tip). Although both contain substances known to be harmful, science is still working to learn more about the long-term effects of exposure to second-hand cannabis smoke.

A recent study involving THC-free smoke and mice has indicated second-hand cannabis smoke constricts blood vessels in a similar way to second-hand tobacco smoke, but it has not been done on humans, and is considered a starting point for further research.

Matthew Springer, Ph.D., the senior author of the study, summed up his early conclusions: "If you're hanging out in a room where people are smoking a lot of marijuana, you may be harming your blood vessels. There's no reason to think marijuana smoke is better than tobacco smoke. Avoid them both."

Have a question about marijuana news or culture in Alaska? Send it to cannabis-north@alaskadispatch.com with "Highly Informed" in the subject line.

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