Cigarettes aren't just toxic when they're being smoked. Even when the butts are scrunched up and cold, new research has found they continue to emit harmful compounds in the air.
In the first 24 hours alone, scientists say a used cigarette butt will produce 14 percent of the nicotine that an actively burning cigarette would. And the airborne emissions don't stop there.
While most of these chemicals are released within a day of being extinguished, an analysis for the United States Food and Drug Administration (FDA) found concentrations of nicotine and triacetin - a plasticiser in cigarette filters - had fallen by just half a whopping five days later.
"I was absolutely surprised," says environmental engineer Dustin Poppendieck from the United States National Institute of Standards and Technology (NIST).
"The numbers are significant and could have important impacts when butts are disposed of indoors or in cars."
While much attention has been paid to the health impacts of first-hand, second-hand and now third-hand smoking - where chemical residues linger on walls and furniture - the actual butt of the matter has been paid little attention.
To measure airborne emissions from this forgotten remnant, Poppendieck and his team placed 2,100 recently extinguished cigarettes inside a walk-in chamber made of stainless steel.
Of course, they didn't smoke all these themselves. Instead, the team built a machine to take six puffs per cigarette, mimicking the behaviour of actual humans in robotic-like movements.
Once the freshly-extinguished ends were sealed away, the team measured eight chemicals commonly emitted by cigarettes, four of which the FDA have their eye on for being harmful or potentially so.
Triacetin is not one of the dangerous ones, but because it's so common in cigarette filters and doesn't evaporate easily, it's a good indicator of how other sticky chemicals are breaking down.
Fiddling with the room's temperature, humidity and saturation, the authors tested how emissions changed under certain conditions. When the air temperature of the room was higher, for instance, they noticed the butts emitted these chemicals at higher rates.
In other words, leaving ashtrays out for days at a time, especially in the heat seems like a bad idea, and could potentially expose smokers and nonsmokers to even more harmful chemicals than we thought.
"Hence," the study authors conclude, "the emitted nicotine mass from a butt over five days could be comparable to the nicotine mass emitted from mainstream and sidestream smoke, especially at higher temperatures."
The findings are limited because they only implicate one leading brand of cigarettes and there are few other studies to compare them to. Nevertheless, if the numbers are accurate, it means we've been overlooking a big factor of cigarette smoking.
Today, it's estimated that more than five trillion butts are produced worldwide each year, and many of these don't get fully extinguished or thrown away properly, causing ground pollution as well, since cigarette butts are not biodegradable.
"You might think that by never smoking in your car when kids are present, you are protecting the nonsmokers or children around you," Poppendieck says.
"But if the ashtray in your hot car is full of butts that are emitting these chemicals, exposure is happening."
The studies were published in the International Journal of Indoor Environment and Health and Science of the Total Environment.