Substances with exposure limits below 100 ppm are considered highly toxic by inhalation. Those substances with exposure limits of 100 - 500 ppm are considered moderately toxic by inhalation. Substances with exposure limits greater than 500 ppm are considered slightly toxic by inhalation. There may be no more than 4 STEL periods in a work day and there must be 60 minutes between the periods. This is the air concentration that triggers a series of actions the employer must take to protect the employee.
Action levels are typically one half of the permissible exposure limit.