In hotels that allow smoking in some rooms, nicotine residue ends up in all rooms, even those labeled “non-smoking,” a new study finds. Partial bans do not protect occupants of non-smoking rooms from tobacco pollution exposure, the researchers report in the journal Tobacco Control.

Smoke-free hotels are becoming more common, USA Today reports. Marriott, Westin and Comfort Inn are among the chains that are now smoke free.

Researchers at San Diego State University found smoke residue on surfaces and in the air of both smoking and non-smoking rooms in a study of 10 California hotels with smoking bans and 30 with partial bans. Non-smoking study participants who spent the night in the hotels provided urine and finger wipe samples, so the researchers could assess their exposure to nicotine.

In hotels operating partial smoking bans, surface nicotine levels and air 3EP (pollution) levels were higher in both non-smoking and smoking rooms, compared with hotels with total smoking bans. Surface nicotine levels were more than twice as high in non-smoking rooms of hotels with partial bans, compared with hotels with total bans. Air levels of 3EP were more than seven times as high.

Air nicotine levels were 40 percent higher in non-smoking rooms of hotels with partial bans, compared with those in hotels with total smoking bans. Non-smokers staying in hotels with partial bans had higher levels of nicotine on their fingers and in urine, compared with non-smoking guests staying in hotels with total bans.

“Our findings demonstrate that some non-smoking guest rooms in smoking hotels are as polluted with [third hand smoke] as are some smoking rooms,” the authors note in a news release. “Moreover, non-smoking guests staying in smoking rooms may be exposed to tobacco smoke pollutants at levels found among non-smokers exposed to second hand smoke.”