In addition to launching a projectile, firing a gun also releases particles and gases (some of which condense into particles after cooling down) into the air. Collectively known as gunshot residue, these airborne substances are derived from parts of the cartridge (bullet/shot + propellant + primer + case) and the firearm itself. Gun-sourced particles settle out onto nearby surfaces, including the hands, hair, and clothing of the shooter. Forensic analysis of gunshot residue involves sampling these surfaces (e.g. by applying the sticky side of a piece of Scotch tape and then lifting it off) and then determining if residue is present by looking at the shape of sampled particles as well as the presence and concentration of certain elements (e.g. antimony, barium, and lead, which are found in some bullets) and organic compounds. It enables folks in law enforcement to determine if a person was near a gun when it was fired, and can also potentially provide information about the particular gun and/or type of ammunition used.
Where am I going with all of this? It turns out people sweat, frequently touch all sorts of things, and also sometimes wash their hands or take showers. These actions result in the removal of gunshot residue from their skin and hair. Given this, you generally have less than half a day to find gunshot residue on a person's hands after they fire a gun. Yet there's another place the residue can be found: nasal mucus!
|There's gold (and maybe ammo fragments) in them hills! (Source)|
When you breathe in through your nose, larger particles in the inhaled air become trapped in the mucus coating the inside of your nasal cavity. It's sort of an inverted immobile lint roller, if you will. The adhesion of particles to snot is great because they might otherwise end up deep inside your lungs where they can potentially cause problems. It turns out gunshot residue can stick to nasal mucus, so it's possible to take samples of snot and analyze them in order to identify a shooter. Although less residue ends up in nasal mucus than on hands, it tends to stick around for longer. Snot-based residue is also specifically associated with a gun being fired, as opposed to it being handled after the fact by someone who didn't fire it.
One of the earliest investigations into the use of nasal mucus to detect gunshot residue involved having people blow their nose into a sheet and then analyzing the boogers. This approach was problematic since it required a person to not have blown their nose recently. It also tended to cause nosebleeds. A newer means of sampling is to rub a moistened cotton swab around the inside of both nostrils. The swab is then placed in a tube filled with an acid or other liquid to pull out the substances of interest, and the resulting solution is analyzed.
Aliste M, Chávez LG. 2016. Analysis of gunshot residues as trace in nasal mucus by GFAAS. Forensic Science International 261:14-18.
Chang KH, Jayaprakash PT, Yew CH, Abdullah AFL. 2013. Gunshot residue analysis and its evidential values: A review. Australian Journal of Forensic Sciences 45(1):3-23.