18. An atmosphere is considered oxygen deficient and IDLH when oxygen levels fall below __ percent. (485) 15.6 19.5 20.9 23.5

## Step 1<br />The problem is asking for the oxygen level at which an atmosphere is considered oxygen deficient and Immediately Dangerous to Life or Health (IDLH). The options provided are 15.6, 19.5, 20.9, and 23.5.<br /><br />## Step 2<br />The term "IDLH" is used to describe conditions that pose an immediate threat to life, cause irreversible adverse health effects, or hinder the ability to escape from a dangerous environment.<br /><br />## Step 3<br />According to the Occupational Safety and Health Administration (OSHA), an atmosphere is considered oxygen deficient and IDLH when oxygen levels fall below 19.5 percent.