Methods to Lower Workplace PELs

According to OSHA policy, engineering controls and work practices are preferred over personal protective equipment to control employee exposures to airborne contaminants.

Engineering controls involve the use of a local exhaust ventilation, general ventilation, isolation of the worker and enclosure of the source of emissions, process modifications, equipment modifications, and substitution of nonhazardous or less hazardous chemicals. These methods may be used alone or in combination, depending upon the industrial processes involved. These controls are widely used and will effectively control exposures either by themselves, or coupled with changes in work practices.

Ventilation

Perhaps the most widely used technique for controlling chemical exposure is the use of ventilation. General ventilation uses the movement of air within the general work space to displace or dilute the contaminant with fresh outside air. General ventilation may not be the preferred control method, however, due to the large volumes of air movement required. Local exhaust ventilation uses a much smaller volume of air and controls emissions at the point or source from which contaminants are generated.

Isolation

Isolation involves placing a physical barrier between the hazardous operation and the worker. Many modern, automated manufacturing processes are now fully enclosed in ventilated cabinets. The effectiveness of such a control technique depends on the frequency with which the workers have to enter the enclosure during normal operations.

In other situations, the worker, rather than the process or machine, can be placed in an enclosure having a controlled atmosphere. Many processes which involve potential chemical exposures are operated remotely by operators from air conditioned booths isolated from the hazardous materials.

Substitution

Substitution refers to the replacement of a toxic chemical in a particular process or work area with another, less toxic or non-toxic product. Properly applied, substitution can be a very effective control technique. However, care must be taken to ensure that the proposed substitute performs in a similar manner to the product being replaced. In addition, it is essential that the substitute be carefully evaluated to ensure that in controlling one hazard, another different hazard is not inadvertently introduced. The substitute must also be compatible with existing manufacturing equipment and processes.

The success of these engineering control techniques will depend on the physical properties of the chemicals and emissions encountered (boiling point, vapor pressure, etc.) and the process operating conditions. In some cases, particularly with cleaning solvents, substitution may provide the quickest and most effective means of reducing exposure. In other situations, a major effort may be required to alter processes or install or expand local or general dilution ventilation.

OSHA has found that engineering controls and improved work practices are available to reduce exposure levels to the new levels in almost all circumstances. However, in some circumstances, respiratory protection may be necessary to complement engineering controls. Respiratory protection may be necessary to achieve compliance in some specific operations in some industries.

Engineering Controls vs. Protective Equipment

So in other words, the most desirable way to deal with an air contaminant is to alter the process so that the contaminant is no longer produced. If the process cannot be changed or materials substituted, a well designed ventilation system may be the best solution to the problem. If ventilation would require too large a volume of air to reduce the concentration of the contaminant, then respiratory protection may be a necessary short-term solution.

Pitfalls of Exposure Limits

Even though you have checked the PELs and the TLVs for your chemicals, brought the exposure numbers into the acceptable range, your workers may still be endangered. How could this be?

There are over 600,000 chemicals in use today. Information available for selecting an exposure limit is very scant. Only a small percentage of chemicals is even evaluated. Therefore, supporting data can be weak. Exposure limits are changed when new information becomes available. What is considered “safe” today, may be viewed in a different light tomorrow.

Individual sensitivity is a factor. Even at an acceptable exposure level, a given chemical may have a negative effect on certain people. So even if the exposure limit protects most people, it can not be relied upon to protect everyone.

Synergistic effects should be considered. Single substances are assigned individual ratings. Seldom in the real world is only one chemical in use at a time. What happens when several chemicals combine to produce effects far more harmful than those of any one substance?