Air conditioner
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Thirdhand smoke can be picked up and spread around buildings by HVAC (heating, ventilation, and air conditioning) systems, NBC News reported.

The report noted that exposure does not only come from secondhand smoke, but also from people smoking outside a building or who smoked in the building years ago.

Drexel University's Peter DeCarlo wrote, "For people who do not smoke and avoid areas where smoking occurs, this is an additional involuntary exposure route for environmental tobacco smoke species."

In an experiment conducted in a room where no one had smoked for years, DeCarlo found that 29% of the air particles were affected by tobacco components. Though it's not clear how the particles entered the room, DeCarlo and his colleagues noted that "20 meters down the hall" is an "outdoor balcony where illicit smoking occurs," NBC emphasized.

They also noted that HVAC systems recirculate air throughout multiple rooms, so "what happens in one room affects all the other rooms in the zone."

"For this reason, a room located near a smoking area with smoke penetration or a room occupied by a smoker can effectively expose the other occupants served by the same HVAC system to thirdhand smoke, even if they do not share space directly."

According to the Center for Disease Control (CDC), at least 2.5 million non-smokers have died from secondhand smoke since 1964, NBC addded.

In 2017, a research showed that children placed in a room where people had previously smoked but were no longer smoking in had significant amounts of nicotine on their hands and in their saliva.

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