Why do airplane pilots wear film badges?

The film badges (radiation dosimeters) indicate if a pilot has been exposed to higher levels of ionizing radiation, as from cosmic rays at high altitudes, or from other sources. Limiting the periodic exposure prevents unsafe effects on the flight personnel. Most US pilots now use a different device, a thermoluminescent detector or TLD. So for short: Because they work at high altitudes, pilots are exposed to high levels of background radiation from cosmic rays. To monitor their radiation exposure, pilots wear film badges.