What kind of smoke is used in mask fit testing?

Mask fit testing is a procedure used to ensure that the respirator mask selected for use during work fits comfortably and securely in order to properly protect you from airborne hazards. For respirator masks that employ a face-to-mask seal, either a qualitative fit test (QLFT) or quantitative fit test (QNFT) must be conducted to assure the efficacy of the seal. Occupational Safety and Health Administration (OSHA) standards for mask fit testing designate four non-toxic substances which may be used as contaminants for qualitative testing.

The irritant smoke contaminant used to determine the adequacy of a respirator mask's seal is created using stannic chloride or stannic oxychloride. Other substances used for qualitative testing include isoamyl acetate (banana oil), saccharin, and denatonium benzoate (Bitrex).

Stannic chloride (SnCl4) is a colorless liquid with a noxious odor. When stannic chloride is combined with water vapor (H2O), the chemicals stannic oxychloride (SnOCl2) and hydrogen chloride (HCl) are formed. Hydrogen chloride, when mixed with water vapor in the air, forms a visible smoke. For the irritant smoke test, stannic chloride is passed through a ventilation smoke tube where it combines with water vapor forming the two chemical byproducts and subsequent irritant smoke.

The chemicals produced by the stannic chloride ventilation smoke tube irritate a person's eyes, nose, and lungs. However, individual responses to this chemical irritant do vary. Because of this, OSHA mask fit testing protocols require test operators to conduct a low-exposure sensitivity test for your safety before the full mask test is performed. Once your sensitivity to the stannic chloride smoke has been assessed, you will be asked to put on the test respirator and will be exposed to a greater concentration of the smoke. If the respirator mask's faceplate seal is inadequate, you will be able to detect the smoke and may experience an involuntary response such as coughing. If you show no reaction to the smoke, final testing is conducted to confirm the results. Upon confirmation, this portion of the mask fit test is complete.