Secondhand marijuana smoke could damage heart and blood vessels as much as secondhand tobacco smoke, according to a recent study.

Researchers found that secondhand marijuana smoke may have similar cardiovascular effects as tobacco smoke. Lab rats exposed to secondhand marijuana smoke had a 70 percent drop in blood vessel function.  

Marijuana and tobacco smoke are chemically and physically alike, aside from their active ingredients.

"Most people know secondhand cigarette smoke is bad for you, but many don't realize that secondhand marijuana smoke may also be harmful," Matthew Springer, senior author of the study and cardiovascular researcher and associate professor of Medicine at the University of California, San Francisco's Cardiology Division, said in a statement.

For the study, researchers used a modified cigarette smoking machine to expose rats to marijuana smoke. A high-resolution ultrasound machine measured how well the main leg artery functioned. Researchers recorded blood vessel dilation before smoke exposure and 10 minutes and 40 minutes after smoke exposure.

They also conducted separate tests with tetrahydrocannabinol-free (THC-free) marijuana and plain air. There was no difference in blood vessel function when the rats were exposed to plain air.

In the study, blood vessel function in lab rats dropped 70 percent after 30 minutes of exposure to secondhand marijuana smoke. Even when the marijuana contained no THC -- a compound in marijuana that produces intoxication -- blood vessel function was still impaired.

Now that marijuana is becoming increasingly legalized in the United States, its effect on others is a growing public health concern, Springer said.

"If you're hanging out in a room where people are smoking a lot of marijuana, you may be harming your blood vessels," he said. "There's no reason to think marijuana smoke is better than tobacco smoke. Avoid them both."

Secondhand tobacco smoke causes about 34,000 premature deaths from heart disease each year in the United States among nonsmokers, according to the U.S. Surgeon General's 2014 report on the consequences of smoking.

More research is needed to determine if secondhand marijuana smoke has other similar effects to secondhand cigarette smoke in humans.

The preliminary research was presented at the American Heart Association's Scientific Sessions 2014.