A growing number of American parents are using marijuana when they still have children living at home, according to a new study that suggests cannabis may be complicating efforts to limit kids’ exposure to second-hand smoke.

Researchers examined data collected from 169,259 U.S. adults from 2002 to 2015. During that time, the proportion of parents with children at home who said they used cannabis at least once in the past month rose from 4.9 percent to 6.8 percent.

• Over that same period, the proportion of parents with kids at home who smoked cigarettes declined from 27.6 percent to 20.2 percent, the study also found.

“While cigarette smoking continues to decline among parents with children living at home, use of cannabis is increasing among parents and this may as a result lead to an increase in children’s exposure to secondhand cannabis smoke,” said lead study author Renee Goodwin of the Graduate School of Public Health and Health Policy at the City University of New York.

The increase in cannabis use appeared to be “disproportionately common among cigarette smoking parents,” Goodwin said by email. “Therefore we may be seeing an increase in exposure to multiple types of smoke/increased amount of smoke in a growing percentage of households with this increase in cannabis use.”

With some forms of marijuana now legal in about 30 U.S. states, concern is mounting in the medical community that many people may falsely assume the drug is harmless and fail to recognize the potential harms to children who breathe second-hand smoke.

Cannabis use was almost four times more common among parents who also smoked cigarettes than among non-smokers, the current study found.

Among smokers, the proportion of parents who reported using cannabis in the past month increased from 11 percent to 17.4 percent during the study period, researchers report in Pediatrics.

For non-smokers, the proportion of parents who used cannabis in the past month also rose, from 2.4 percent to 4 percent.

Daily cannabis use also climbed during the study period, and was more common among cigarette smokers.

At the same time, the proportion of parents who said they avoided both cigarettes and cannabis also increased.

The study wasn’t a controlled experiment designed to prove whether or how cigarette smoking might influence cannabis use. Another drawback is that researchers relied on parents to accurately recall and report any tobacco or cannabis use.

When parents with young kids at home do choose to use cannabis, they should take precautions to do it when children aren’t around, Brooks-Russell advised.