Fit Tests: How do you know your mask is good enough for the job?

In a previous article we looked at the difference between quantitative and qualitative fit tests. From that article we know that quantitative fit testing is far more effective, and the TSI Portacount is the most widely used quantitative testing device.

But how can this help to determine which type of mask should be worn for a job? For that you need to know a few numbers, and first among them is the Fit Factor.

With qualitative testing the Fit Factor is either 0 (fail) or 100 (pass). Because the Portacount is a much more sensitive piece of equipment the Fit Factor can be more specific, and much higher than 100.

The Portacount works by measuring the amount of a particulate both outside and inside the mask. Any particulate found inside the mask must have have seeped in from outside. The ratio of particulate outside the mask to the amount inside the mask is the fit factor.

Along with the Fit Factor there is also the Assigned Protection Factor (APF). OSHA assigns the APF number to each mask. For example, an N95 mask has an APF of 10. A full face mask would have an APF of 50.

To consider it an appropriate fit the Fit Factor must be at least ten times the APF. In cases where this number is higher than 100 it is necessary to carry out a quantitative test.

Finally, two other numbers that factor in to finding the appropriate mask are the Permissible Exposure Limits (PEL), which is assigned to the chemical by the relevant regulatory body, and the Maximum Use Concentration (MUC), or the maximum amount of a substance a worker can use.

You can calculate the MUC by multiplying the PEL and the APF together. For example if you are working with Acetone, which has an OSHA assigned PEL of 1000, and were using an N95 mask, with an APF of 10, the MUC would be 10,000, which is 24,000mg/m3.

When dealing with a more noxious chemical, for example Bromine, with a PEL of 0.1, using an N95 would only allow the use of 7 mg/m3 of the substance. To use more would require a heavier mask.

Your local Occupational Health and Safety organization should publish a guide to acceptable exposure limits.