A new study by researchers from the University of California-San Francisco claims exposure to secondhand marijuana smoke may be just as damaging to health as exposure to secondhand cigarette smoke.

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In rats exposed to secondhand marijuana smoke for 30 minutes, their blood vessel function reduced by 70%.

The research team, including senior author Matthew Springer, an associate professor of medicine in the Department of Cardiology at UCSF, present their findings at the American Heart Associations's Scientific Sessions 2014 in Chicago, IL.

"Most people know secondhand cigarette smoke is bad for you," notes Springer, "but many don't realize that secondhand marijuana smoke may also be harmful."

For their study, the researchers demonstrated the health effects of secondhand marijuana smoke by exposing laboratory rats to it using a modified cigarette smoking machine.

The team used a high-resolution ultrasound machine to measure the rats' blood vessel function 10 minutes before exposure and 40 minutes after exposure.

After being exposed to the secondhand marijuana smoke for 30 minutes, the researchers found that the blood vessel function of the rats reduced by 70%.

The team then exposed the rats to secondhand smoke from marijuana that did not contain tetrahydrocannabinol (THC) - the drug's active ingredient. They found that 30 minutes of exposure to this smoke also reduced blood vessel function in the rats, indicating that the effect is not down to THC.

As a comparison, the researchers assessed the effects of secondhand exposure to THC-free marijuana smoke and plain air. Rats experienced no change in blood vessel function when exposed to plain air.

The researchers note that secondhand exposure to tobacco smoke is known to reduce blood vessel function, but this effect usually wears off within 40 minutes. In this study, however, the blood vessel function of rats exposed to secondhand marijuana smoke had not returned to normal after 40 minutes.

'No reason to think marijuana smoke is better than cigarette smoke'

The team believes their results are worrying. Impaired blood vessel function can increase the risk of heart attack and atherosclerosis - a major risk factor for heart disease.

They also point out that more and more states in the US are legalizing marijuana. As such, Springer says that the effects of exposure to secondhand marijuana smoking is an increasing public health concern. He adds:

"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."

Research looking at the health effects of marijuana has been mixed. Last year, Medical News Today reported on a study suggesting that chemical compounds found in marijuana - THC and cannabidiol (CBD) - could protect the nervous system against multiple sclerosis. A more recent study claims that THC could be used to reduce tumor growth in patients with cancer.

However, many studies have documented the negative effects that marijuana may have on health. In December last year, we reported on a study suggesting that marijuana may lead to poor memory and changes in brain function similar to those found in patients with schizophrenia, while other research found that marijuana may cause sleep problems.