Before firing into a crowded movie theater in Aurora, Colorado, the gunman now identified as James Holmes reportedly set off a canister of smoke, CNN reports.
Twelve people were killed, the AP reports, and at least 38 others were injured.
A number of news outlets report that some of the injured are being treated for chemical exposure, most likely tear gas.
Tear gas -- the common name used for any of a number of chemicals, including chloroacetophenone (CN), chlorobenzylidenemalononitrile (CS) and dibenzoxazepine (CR) -- is used as a riot control agent, according to the New York City Department of Health's website. These chemicals cause temporary irritation of the mouth, throat, lungs and skin, and, of course the eyes, which water profusely, giving the chemicals their name: tear gas.
According to the NYC DOH, the effects usually fade after 30 or 60 minutes after washing the chemicals off, but personal anecdotes tell of longer-lasting concerns. A tear gas researcher who has experienced the chemical first-hand told the Pittsburgh Post-Gazette of lingering irritation that affected his eyes and nose for days after exposure.
However, there is little evidence to support physical problems from tear gas exposure persisting beyond a few days, according to a small study published in the Journal of the Royal Society of Medicine in 2003. But tear gas can be particularly dangerous for people with asthma or other existing health concerns related to the lungs, just as something non-toxic, like pollen, can exacerbate these conditions.
CS, the most commonly used tear gas, is a powder in its natural form, but can be mixed into an aerosal spray, a dust or a smoke propelled from canisters that explode into thick clouds, the Post-Gazette reported.
If Holmes didn't use tear gas, chemical smoke of another kind could still pose health risks -- especially to people with existing respiratory concerns. However, there is also more evidence for long-term effects of smoke inhalation in previously healthy people.' Inhaling smoke blocks the body from taking in all the oxygen it needs, the BBC reported. Carbon monoxide or other toxic chemicals can be inhaled and damage the lungs, but even just carbon dioxide can take up the space the body reserves for oxygen, according to WebMD.