Despite decades of knowledge about the dangers of tobacco, almost 67 million Americans still smoke cigarettes or use other tobacco products. Nearly 42,000 deaths are caused by secondhand smoke, including nearly 34,000 deaths from heart disease.

Why does smoke cause heart disease?

Matthew Springer, professor of medicine at the University of California, San Francisco, told Healthline that healthy arteries sense increases in the amount of blood being pushed through them, and they respond by vasodilating, or widening.

“This enables muscles, including the heart, to get enough blood and oxygen when they are working more, and it prevents atherosclerosis. Tobacco smoke (both inhaled by smokers and secondhand smoke inhaled by bystanders) temporarily impairs this response. If someone is exposed frequently enough to tobacco smoke, their response is reduced even if they aren't being exposed to smoke during the test,” he said.

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Marijuana Versus Tobacco

In his study, Springer obtained marijuana cigarettes from a National Institute on Drug Abuse (NIDA) contractor. He placed these marijuana cigarettes into a puffing machine, which simulates human smoking, and separately gathered the main stream of “inhaled” smoke as well as the side streams of secondhand smoke.

Using a concentration of marijuana smoke comparable to the levels of secondhand tobacco smoke found in bars and restaurants, Springer exposed sleeping rats to the smoke for 30 minutes, then measured their blood vessel function.

The rats exposed to the secondhand marijuana smoke showed the same reductions in blood vessel function as rats exposed to secondhand tobacco smoke. “More notable is that in the case of tobacco, we saw impaired blood vessel function after a 30-minute exposure had returned to normal when measured 30 minutes later; but in the case of marijuana, the function had not recovered 30 minutes later,” Springer said. “We have not determined how long the impairment lasts.”

When Springer exposed the rats to marijuana smoke without THC, an active component of marijuana, he saw the same results. The smoke itself was to blame.

One possible limitation of the study is that scientists don’t yet know how concentrations of secondhand marijuana smoke created by social users compare to secondhand tobacco smoke. “Someone in a room where people are smoking marijuana might get a different level of smoke. We can only say that smoke is smoke. Notably, we published earlier this year that modest but significant impairment [of blood vessel function] can be detected after just one minute of exposure to tobacco secondhand smoke at similar levels,” said Springer.

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Marijuana Smoke Has Twice the Tar, 20 Times the Ammonia

Another question raised about the findings is the quality of the marijuana itself. Medical marijuana users tend to exclusively use sinsemilla, the unfertilized flowers of the female plants. However, marijuana products can more efficiently be made from whole male and female plants, which include a higher proportion of stems and leaves. The additional presence of leafy material might produce additional toxins.

For Springer's study, this wasn't a problem. He received his marijuana supplies from a NIDA contractor who works with the University of Mississippi. According to Mahmoud ElSohly, a researcher professor at the University of Mississippi, the marijuana supplied for research is composed entirely of sinsemilla. ElSohly added, "Whatever plant material you have (buds, leaves, etc.) is plant material. The part that is important is the [chemical] constituents."

Research published in Chemical Research in Toxicology compared tobacco smoke with marijuana smoke made from sinsemilla. The two types of smoke had similar profiles, except for the absence of nicotine in the marijuana. Also, chemicals such as carbon monoxide, formaldehyde, acrolein (used in some types of tear gas), toxic heavy metals, and a wide range of carcinogenic organic compounds are formed during the combustion of the tobacco. The secondhand marijuana smoke, however, had almost twice as much tar as the tobacco smoke, as well as 20 times as much ammonia (possibly from hydroponic fertilizer), and over twice as much hydrogen cyanide, which is a highly poisonous gas or liquid.

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Changing Minds, Saving Hearts

Springer hopes his research will influence the public’s perception that secondhand marijuana smoke is as harmful as secondhand cigarette smoke. “If you avoid cigarette secondhand smoke, avoid marijuana secondhand smoke also,” he advised.

“Public smoking bans in many cities around the world have led to reduced incidence of heart attacks. With recreational marijuana being increasingly legalized, there will be a new source of openly public secondhand smoke exposure. Laws and policies that limit public smoking should be written widely enough that they encompass marijuana as well as tobacco cigarettes and cigars; and they should be enforced,” Springer concluded.

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