A perfectly sealed manhole has a volume of 10 cubic meters. It contains normal atmospheric air (20.9% oxygen, roughly 79% nitrogen). A leaking pipe introduces exactly 1 cubic meter of pure nitrogen gas, pushing out 1 cubic meter of the mixed air through a vent, which then seals. Calculate the new percentage of oxygen. Is it safe to enter?

A worker uses a gas-powered saw near a trench. The CO concentration in the trench varies: 100 ppm for 2 hours, 30 ppm for 4 hours, and 10 ppm for 2 hours. Calculate the 8-hour Time-Weighted Average (TWA) and compare it to the OSHA Permissible Exposure Limit (PEL) of 50 ppm.

Hydrogen Sulfide ($H_2S$) has an IDLH (Immediately Dangerous to Life or Health) limit of 100 ppm. A sewer line is opened, and a gas monitor registers an $H_2S$ spike of 0.05% by volume. Does this exceed the IDLH limit?

A worker enters a subterranean vault to perform an inspection. He suddenly collapses. His partner outside the vault, acting on instinct, immediately climbs down the ladder to pull him out. Halfway down, the partner also collapses. A third worker runs over and climbs in, also collapsing. All three perish.

A worker is lowered into a grain silo to break up a "bridge" (a hardened crust) of material that has formed across the top of the grain. They are wearing a fall protection harness attached to a short lanyard. They step onto the crust, it breaks, and they sink into the flowing grain.