NIOSH Air Sampling Methods

The National Institute for Occupational Safety and Health (NIOSH) requires workplace compliance for testing airborne contaminants and protecting worker safety. The sampling methods prescribed must produce results that fall within 25 percent of the true value of the sample 95 out of 100 times. Because air contaminants can take the form of gas, aerosol, vapor or a combination of these states, sampling methods are required to address the ability to recover the sample for suitable analysis, sampler capacity and effects of environmental factors. Commercial samplers are available for use, but a basic understanding of sampling methods is required. Most important, a statistician should be employed to design a random sampling pattern with consideration of exposure periods (i.e., during working hours and shifts).

Exposure Limits

• An exposure limit for any contaminant is the concentration above which worker exposure must be limited or prohibited during work hours due to health risks. Exposure limits take into account the physical and chemical properties as well as the toxicological, and associated health effects of contaminants. Exposure limits set the boundaries for sampling results; generally 0.1 to 2 times the exposure limit is what sampling equipment must be calibrated to monitor. Lower limits are set for contaminants with known carcinogenic characteristics.

Samplers

• Samplers are generally worn on clothing to determine inhalation of contaminants -- that is, personal exposure; samplers are also placed within rooms to identify ambient conditions. Personal samplers must allow freedom of movement while also being capable of adequately sampling particulates, keeping in mind that airborne contaminants are generally unseen. Specific attention should be paid to the materials used in samplers and flow meters to absorb the air particles (the "sorbents"). Different sorbent beds may need to be increased to meet the sampling requirements but may be too cumbersome for personal sampling.

  Another consideration is fortification, where by samplers must be fortified so that the contaminant can be recovered for laboratory analysis. Samplers may be fortified so that the percent of analyte recovered is equal to the amount of analyte found on sampler divided by the amount of fortification equal to 100 percent.

Ambient Conditions

• Sampling methods must take into account ambient conditions within the workplace. Sample generation is affected by temperatures; higher temperatures can decrease ambient concentrations. Ambient pressure conditions also play a role; reduced pressure can reduce sampler capacity. Sampler capacity is a function of flow rate, time-weighted averages and sampling run time, especially if a sample has a short-term exposure limit. Furthermore, relative humidity can either decrease or increase sampler capacity depending upon the sample and equipment chosen. As a general rule, sampling should occur during typical workplace conditions, and repeated samples and adequate sample size for validation are required.