When making hotel reservations it’s a comfort to know you can choose nonsmoking rooms. But don’t be misled. New research shows evidence of nicotine even in rooms that are designated as smoke-free. And with mounting evidence that secondhand smoke exposure can cause serious health problems, those frequently staying at hotels should be aware of the potential risks.

Just published in the journal Tobacco Control, the study involved researchers visiting low- to mid-budget urban hotels. Ten of the establishments allowed no smoking at all, while 30 offered their guests smoking and nonsmoking accommodations.

The investigators then recruited groups of nonsmoking guests at each of the hotels, some who were staying for one night in nonsmoking rooms, and some staying in smoking rooms. Each guest was evaluated for exposure to nicotine and NKK, a carcinogen found in tobacco smoke, through urine and finger swipe samples, and all rooms were checked for residual amounts of both substances.

Not surprisingly, the physical analysis showed surface nicotine levels that were 35 times higher in rooms where previous guests had smoked, and air nicotine levels were 22 times greater than those of rooms in hotels operating under a complete smoking ban. But measurements taken in nonsmoking rooms of hotels with smoking rooms also showed a surface nicotine levels over two times as high, and air levels over seven times as high, as those of hotels with complete bans. This demonstrates that only a total ban can keep the rooms nicotine-free.

The researchers also noted that nonsmoking guests who stayed in hotels with partial bans had higher levels of nicotine compared to guests staying in hotels with total bans.

If you want to stay smoke-free, check your hotel’s policy before your next trip.