When humans return to the Moon and travel to Mars, they’ll have to be careful of what they inhale. In 1972, Apollo astronaut Harrison Schmitt sniffed the air in his Lunar Module, the Challenger. “[It] smells like gunpowder in here,” he said. His commander Gene Cernan agreed. “Oh, it does, doesn’t it?” The two astronauts had just returned from a long moonwalk around the Taurus-Littrow valley, near the Sea of Serenity. Dusty footprints marked their entry into the spaceship. That dust became airborne–and smelly. Later, Schmitt felt congested and complained of “lunar dust hay fever.” His symptoms went away the next day; no harm done. He soon returned to Earth and the anecdote faded into history.