The words immediately dangerous to life or health or IDLH mean that chemicals in the air could be high enough to be bad for a person's health. The high amount of chemicals in the air (their concentration in ppm/ppb) could cause quick injury or death. The concentrations in the United States are set by NIOSH and OSHA. Many equations are used to find the "lethal dose", which is also called LD50: mg substance inhaled per kg body weight. NIOSH then publishes the numbers that show how much of a chemical is bad for humans.
IDLH numbers have been used to help people choose the right respirator.