“Sniffer missions were comprised of having a machine in the cargo compartment behind the pilots that…”

“Sniffer missions were comprised of having a machine in the cargo compartment behind the pilots that would measure ammonia levels in the air. There were generally two guys on board, who operated the ammonia sensing equipment, besides the four man crew. Since congregations of humans gave off a lot of ammonia as a result of their metabolism, the army figured this would be a good way to find groups of enemy troops. The only drawback was that congregations of monkeys also gave off a lot of ammonia. It was Standard Operating Procedure for a Sniffer flight to be flown at fifty feet, just above the tops of the trees, at fifty knots airspeed. In other words, it was dangerous as hell. During the flight, the ammonia machine operators would say “mark!” into the intercom radio and the crew chief or door gunner would throw out a smoke grenade, marking the area. Immediately a gunship fire team would roll in on the smoke-marked area and blow the absolute crap out of it. I always suspected a lot of monkeys were needlessly massacred.”

Guts ‘N Gunships: What it was Really Like to Fly Combat Helicopters in Vietnam by Mark Garrison, page 104