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Employee exposure to hazardous chemicals such as formalin and xylene must be monitored annually

Lab Safety Advisor, February 14, 2006

Exposure Monitoring Methods

Employee exposure to hazardous chemicals such as formalin and xylene must be monitored annually by using a representative sampling strategy and by measuring sufficient exposures within each job classification for each work shift. The critical monitoring process is repeated each time there is a change in production, equipment, process, personnel, or control measures that result in new or additional exposure to hazardous chemicals.

There are many types of exposure monitoring methods available. It is up to management to select the best monitoring device for the application needed. To sample worker exposure, you can use one of the following:

Diffusion tubes

Diffusion tubes can be used to determine either a time-weighted average (TWA) over an eight-hour time period or a short-term exposure limit (STEL) over a 15-minute time period. Diffusion detector tubes meet OSHA guidelines for air sampling, but they are not the most accurate method. The advantage to this type of sampling is the immediacy of the reading and the time spent waiting for laboratory results is eliminated. However, there are often chemical cross sensitivities that can cause error to the reading.

Vapor monitor badges

Vapor monitor badges work by the principle of diffusion. The worker is given the badge, which clips onto the collar. The exposure time, temperature, relative humidity, date exposed, name of employee, and monitor number must be recorded. This information is needed to correctly calculate the exposure level. The badges can be used to determine an eight-hour TWA or a
15-minute STEL.

Some of the badges are sold with a prepaid analysis. All the employer has to do is have the worker wear the badge and then return it in the addressed mailer to the laboratory to be analyzed. Some of the badges can be purchased without the analysis, requiring the analysis to be done by the employer's environmental monitoring laboratory of choice. The results are more accurate than those involving diffusion tubes; however, badges have to be sent away for analysis and results cannot be given immediately. Badges are often more expensive than diffusion tubes.

Air sampling pump

Personal air sampling pumps are a little more difficult to use. The setup consists of a small pump that pulls a constant amount of air (usually in liters per minute), a charcoal tube or filter cassette, and a clip for attaching the collection media near the worker's breathing zone.

These pumps require calibration before each use and must have the flow rate set to the correct level depending on what is being monitored. The worker usually wears the pump and the collection media on his or her person for the entire day. At the end of the day, the charcoal tube or filter cassette is sent to the environmental monitoring laboratory of choice to be analyzed. The results are usually more accurate than other methods; however, use of the pump is more cumbersome than other sampling methods.

Continuous hand-held monitor

Continuous hand-held monitors come in a variety of shapes and sizes. They may vary from a relatively simple single-gas monitor to a complex, data logging four-gas monitor. They all make a quantitative analysis that is displayed on a digital or analog readout. Some also have the capabilities to store information, which can be downloaded onto a computer.

The readout is immediate. There is no waiting for a color change or for a lab to analyze the results of a badge. The monitors also give real-time readout. What is being read on the display is what the gas concentration is at that time. Most hand-held meters also have a visible or audible alarm or both that will alert the user if a gas is above a safe level. However, this type of meter is usually calibrated with one type of gas; interference from similar gases could alter the readings. Compared with other types of monitors, hand-held monitors are often more expensive.

Monitoring on an annual basis is conducted unless the last permissible exposure level (PEL) monitoring results reveal employee exposure at or above the action level at an eight-hour TWA or at or above the STEL over a 15-minute sampling, in which case employee monitoring is repeated every six months until the monitoring falls below the action level or STEL for two consecutive sampling periods at least seven days apart.

When an employee reports signs and symptoms of respiratory or dermal conditions associated with exposure, the employee's exposure is promptly monitored.

Management must review the monitoring results as soon as possible but no more than 15 days after receiving them. The results must be given to the employee for his or her review. When the PEL is above the action level, a written plan to reduce employee exposure is given to each employee. This plan would describe the corrective action being taken to decrease the exposure. In addition, the employee must complete an annual medical disease questionnaire.

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