Workplace Safety Guides

   Controlling Indoor Air Quality


The PEL is OSHA's "Permissible Exposure Limit." The PEL is the highest concentration in air of a chemical substance to which employees may be exposed. The LEL, or "Lower Explosive Limit," is a much higher concentration, at which the chemical gas or vapor will burn or explode if an ignition source is present. Methyl Alcohol, for example, has an LEL of 6.7% in air. This is equivalent to a concentration of 67,000 parts per million more than 300 times higher than its PEL of 200 parts per million.

IAQ is a popular term standing for "Indoor Air Quality." When we talk indoor air quality problems, we're talking about improper temperatures, high or low humidity, odors, oxygen depletion, carbon dioxide build-up, or high concentrations of other air contaminants in the workplace, such as tobacco smoke, biological agents like mold spores, or industrial chemicals.

Normally, the building's heating and air-conditioning systems are expected to control temperatures, humidities, odors, oxygen depletion, carbon dioxide build-up, mold spores, and tobacco smoke. Industrial ventilation systems are often used to control the buildup of hazardous concentrations of industrial chemicals in air.

Where do such airborne chemical contaminants come from? Directly from process equipment, mobile equipment, employee activities, wind and poor housekeeping, and fugitive emissions from industrial process control equipment are some of the sources. As you can imagine, we must identify emission sources before we can effectively control them.

Intensive study of the source is often required before controls are installed. The hazards of emission sources can be controlled by many methods. One good way is to substitute a less toxic material. For example using hot, soapy water in place of solvents for cleaning. Another is to provide a change in the process. For example, using paint dipping instead of paint spraying. Yet another approach is to enclose the source or provide a barrier between the source and the employee. Sometimes worker education or worker placement or other administrative control can reduce emissions and exposures. Approved respirators may be used as a control method in some situations, but usually the most reliable approach to reduce employee exposure is through the use of engineering and administrative controls.

Ventilation control is one such engineering method. This approach has become as industrial ventilation. There are two basic types of industrial ventilation: local exhaust and dilution ventilation. Local exhaust systems capture or contain contaminations at the source before they escape into the work environment. Dilution ventilation systems, on the other hand, allow the contaminants to escape into the work environment. The contaminant concentration is lowered to safe levels by dilution with fresh air before reaching the employees' breathing zone. Where employees are close to the emission source, local exhaust is usually superior to dilution if it is feasible.

Both types of ventilation require tempered, heated, or cooled make-up air to replace air exhausted through the ventilation system. Think about it. Air removed must always be replaced. It is best to do this through designed and installed make-up air systems. Some times the makeup air is provided by building's heating and cooling system.

The local exhaust system consists of five important components: hood, duct, air cleaner, fan, and stack. The hood is the most important part of the system. No local exhaust system will work properly unless enough of the contaminants are retained or captured by the hood so that the concentration of contaminants in the work room air is below acceptable limits.

Duct work or piping transports the contaminants to an air cleaner or to the outdoor environment. Many local exhaust systems must be equipped with air cleaning equipment in order to meet air pollution requirements.

The fan has to develop enough static pressure to pull a desired amount of air and contaminants into the hoods and through the ducts to the point of discharge.

Finally, a stack is used to disperse the exhaust air stream and whatever contaminants get passed in the air clear installed.

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