Dodging the clouds of toxic cigarette smoke along city streets and at building entrances has become the norm for those wanting to avoid the health risks from breathing in second-hand smoke.

But if you think being in a smoke-free building will shield you from the negative health effects of second-hand smoke, you are in for a surprise.

Now there is third-hand smoke: the residue that can be carried on clothing and hair or left behind on couches and counters long after cigarette smoke has cleared. Turns out toxic particles can be activated and released back into the air.

Researchers at Drexel University looked at how particles from outside air get inside and found a "chemical signature" that didn't exist outdoors — which they identified as third-hand smoke, said Peter F. DeCarlo, lead author of the study published Wednesday in Science Advances.

"It was a completely unexpected result," said DeCarlo, an associate professor of environmental engineering and chemistry.

Third-hand smoke particles can become reactivated when they come into contact with the natural ammonia that is emitted from the human body. Indoor temperatures and humidity also play a role in that process, making exposure more of an issue in summer months, he said.

The researchers tested the air of an unoccupied classroom where smoking had not been allowed in decades. They found that 29 percent of the indoor aerosol mass contained third-hand smoke particles. While particulate matter in air is a small fraction of what we breathe in, the worrisome factor was the concentration of contamination, DeCarlo said.

Being in a room that is contaminated with third-hand smoke during an average workday is the equivalent of being in a room filled with second-hand smoke for about five minutes, he said.

Since the building was smoke-free and there was no indication anyone was violating that policy, the researchers looked at possible other sources.

The room was down the hall from an outdoor balcony where people often go to sneak an illicit smoke. It was also part of the same heating, ventilation and air-conditioning zone as a nearby office space shared by several smokers, and the system recirculated contaminated air throughout the building.

Cigarette smoking causes about 480,000 deaths in the United States each year, with more than 41,000 of those deaths attributed to second-hand smoke exposure. Worldwide, there are nearly six million deaths a year related to tobacco use, according to the Centers for Disease Control and Prevention.

The health risks of third-hand smoke are just now becoming more apparent.

In a recent Stanford University study using laboratory mice, researchers found that skin contact with the compounds in third-hand smoke increased the severity of asthma symptoms. It is also known to increase the risk of lung cancer in mice, researchers at the U.S. Department of Energy's Lawrence Berkeley National Laboratory found.

Young children are particularly vulnerable. They are more likely to be crawling on contaminated carpets or ingesting the residue when they put their hands in their mouths.

Concentrations of third-hand smoke particles are likely higher in residences, hotel rooms, or rental cars where people have previously smoked, the Drexel researchers found.

The effects of third-hand smoke need to be part of the discussion when it comes to smoking policies governing areas where the public can be exposed, DeCarlo said.

He pointed to the Philadelphia Housing Authority policy that bans smoking in all living units, indoor common areas, administrative offices, and outdoor areas up to 25 feet from residential and office buildings as a step in the right direction.

In February 2017, the U.S. Department of Housing and Urban Development announced that all public housing must have a smoke-free policy in place by the end of July 2018.

There are some things you can do if your apartment recently converted to a smoke-free unit or if you buy a home previously occupied by a smoker.

The American Nonsmokers' Rights Foundation recommends washing walls and ceilings with detergent and hot water, then applying two to three coats of paint. If the walls are not thoroughly cleaned, nicotine can seep through the new paint. Remove any rugs and padding, then wash floors before replacing carpets. Replace all curtains, blinds or wall coverings. Clean out ventilation ducts and replace all filters.

"'Nonsmoking' really doesn't mean nonsmoking, and that is worrisome," DeCarlo said.