Smoke tubes from companies such as Allegro, Draeger, Gastec, Kwik-Draw® (MSA), RAE and Verifit are an essential OHS tool. Beneficial uses of smoke tubes include respirator fit tests and air flow observations and measurements. Users of smoke tubes must be aware, however, of the significant risks if the tool is misused. Proper use of smoke tubes, therefore, is essential.

Smoke tube operation

Smoke may be generated by a variety of chemical reagents. OSHA, however, specifies that only smoke generated by stannic chloride may be used for respirator fit tests. Most smoke tubes consist of a sealed glass tube containing stannic chloride. When the ends of the glass tube are opened (broken off) and moist air is drawn through the tube by attaching a rubber bulb to one end of the tube, white “smoke” comprised of stannic oxychloride and hydrogen chloride (HCL) is emitted out the other end with each squeeze of the bulb. High relative humidity generates the most smoke. The bulb may be squeezed several to many times to generate smoke before the stannic chloride is depleted.

Respirator fit tests

OSHA 1910.134 App A – Fit Test Procedures (Mandatory) requires that respirators be fit tested by either qualitative or quantitative methods. The four OSHA approved qualitative methods are: (1) Isoamyl Acetate Protocol; (2) Saccharin Solution Aerosol Protocol; (3) BitrexTM (Denatonium Benzoate) Solution Aerosol Qualitative Fit Test Protocol; and, (4) Irritant Smoke (Stannic Chloride) Protocol.

The first three qualitative fit test methods require a subjective response from the employee. Can the employee smell banana, taste sweetness, or detect a bitter taste while wearing the respirator? A “yes” response means the respirator does not fit properly and may not protect the employee from known airborne contaminant(s).

The problem with these first three methods is that they are subjective. The employer can never be 100% sure that the employee’s response was honest. Reasons for a dishonest response include that a poor respirator fit may disqualify the employee from work.

The fourth qualitative fit test method, Irritant Smoke (Stannic Chloride) Protocol, is not subjective. When the test is administered properly i.e. carefully follow each step of the OSHA protocol, particularly achieving minimum time e.g. one minute for each test exercise, it provides an objective involuntary response that the irritant was detected.

In environments where a good respirator fit must be certain, such as Hazwoper and carcinogen exposures e.g. asbestos, irritant smoke should be the preferred qualitative fit test method.

It is not smoke!

OSHA calls the reaction of stannic chloride with moist air as “irritant smoke.” Manufacturers in turn market respirator fit test products as “smoke tubes.” The chemical reaction between stannic chloride and moist air generates a visible plume, however, that is best described as a fume comprised of various particle sizes. In OHS technical terms, smoke is generated from something that burns – and smoke contains not just particulates but gasses, too, such as carbon monoxide.

NIOSH certifies filtering respirators under 42 CFR 84 as being approved against “dusts, fumes, and mists.” Employees that pass an “irritant smoke” respirator fit test are not qualified to use that respirator in an environment with smoke. Initial and annual required Respiratory Protection Training and Information at 1910.134(k) should ensure that employees understand OHS technical definitions for airborne contaminants in forms of dust, fume, mist, vapor, gas, smoke, etc.

Stannic chloride

OSHA specifies that only smoke tubes that contain stannic chloride may be used for qualitative respirator fit tests. Stannic chloride is like nearly every chemical, endocrine disrupting chemicals being an exception, where “dose makes the poison.”

Considering dose, stannic chloride has a special place in history. Lethal chemical weapons during World War I such as chlorine, phosgene and mustard gas constituted war crimes. Stannic chloride, also known as tin (IV) chloride and tin tetrachloride, was first used as non-lethal but disabling chemical weapon by Allied forces in 1916. Make sure that your company’s initial and periodic e.g. annual OSHA HazCom training includes information on stannic chloride smoke tubes.

NIOSH fit test recommendation

Another concept of dose: A 1993 NIOSH study concluded there is a health risk when irritant smoke is used to qualitatively fit a respirator. The study found that at a relative humidity at 53%, HCL reached 11,900 ppm at two inches from smoke tube outlet. A health risk surely exists at that dose – eye and upper respiratory system irritation will be immediately intolerable. NIOSH found 100 ppm HCL at six inches from the tube outlet. But measurements at 12 inches from the tube outlet did not detect HCL.

Because of the potential HCL health risk, NIOSH recommends quantitative fit tests, rather than qualitative irritant smoke, should be used where a high assurance of good respirator fit test is needed. Strict adherence, however, to OSHA Irritant Smoke Protocol manages the risk. Test operators “shall begin at least 12 inches from the facepiece” and gradually move smoke to within six inches around the perimeter of the mask, per OSHA.

Ventilation observations and measurements

Much can be written about using smoke tubes used for ventilation observations and measurements. Visible smoke travels with the speed, direction, and pattern of the invisible air flow. These observations may be used by OHS pros for many purposes such as placement of air monitoring equipment to determine worst case exposures, observe air capture by ventilation exhaust systems or visualize exhaust capture disruption caused by comfort fans, open doors or open windows.

A skilled OHS pro can measure time (seconds) and distance that smoke travels before dissipating to determine feet per minute (fpm). Accuracy rivals velometers and anemometers measurements, particularly at air flows less than 100 fpm. Fpm and cross-sectional area in square feet is used to calculate cubic feet per minute and even room air exchanges per hour.


Operator skills and demeaner e.g. tubes are not a toy are essential for irritant smoke respirator fit tests or for ventilation measurement purposes. Smoke tubes are an essential tool that should be in every OHS’s toolbox.