nds to induce cancer in experimental animals and the ability of methyl chloride to induce teratogenicity in exposed mice. Consequently, appropriate engineering and work practice controls should be used to reduce worker exposure to the fullest extent feasible. The areas in which these monohalomethanes are produced or used should be restricted to only those workers who are essential to the process or operation. The guidelines and recommendations which follow are general in nature and should be adapted to specific situations as required.
Initial and periodic worker exposure surveys should be made by qualified industrial hygiene and engineering personnel. These surveys are necessary to determine the extent of worker exposure and to ensure that controls already in place are operational and effective. NIOSH's Occupational Exposure Sampling Strategy Manual may be helpful in developing appropriate strategies to monitor worker exposure to these monohalomethanes.56 The manual discusses how to determine the need for exposure measurements and to select sampling times.
Worker exposures should be estimated by 8-hour TWA and short-term (15-minute) exposures calculated from personal or breathing zone samples. Short-term samples should be taken during periods of maximum expected exposure by using all available knowledge of the work areas, procedures, and processes. Area and source measurements may be useful in identifying problem areas, processes, and operations.
Detailed descriptions of sampling and analysis techniques for methyl chloride, methyl bromide, and methyl iodide may be found in the NIOSH Manual of Analytical Methods, Second Edition.57
Controlling Worker Exposure
Proper maintenance procedures, good housekeeping in the work area, and worker education are all vital aspects of a good control program. Workers should be informed of the materials to which they are exposed, the nature of their hazard, the methods for their control, and appropriate personal hygiene procedures. There are four basic methods of limiting worker exposure to these monohalomethanes, none of which is a simple industrial hygiene or management decision. Careful planning and thought should be used prior to implementation.
When feasible, substitution of an alternative material with a lower potential health risk is an important method for reducing exposure. Extreme care must be used when selecting substitutes. Possible health effects from potential exposure to alternatives for these monohalomethanes should be fully evaluated prior to selection.
Engineering controls should be used to eliminate the potential for monohalomethane exposure in the workplace and to prevent fires and explosions. Achieving and maintaining reduced concentrations of airborne monohalomethanes in the workplace depend upon the implementation of engineering control measures, such as properly constructed and maintained closed system operations and ventilation, with appropriate safety designs.
Closed system operations provide the most effective means for minimizing worker exposures to these monohalomethanes. Closed system equipment should be used for manufacturing, storing, and processing these monohalomethanes because of their volatility. Where closed systems cannot be employed or do not effectively control monohalomethane emissions, local exhaust ventilation should be provided to direct vapors and gases away from workers and to prevent the recirculation of contaminated exhaust air. Exhaust ventilation systems for quality control laboratories or laboratories where samples are prepared for analyses should be designed to adequately capture and contain monohalomethane vapors or gases. Special consideration should be given to the releasing of these compounds from pressurized sampling containers. Guidance for designing local exhaust ventilation systems can be found in Recommended Industrial Ventilation Guidelines,58 Industrial Ventilation--A Manual of Recommended Practice,59 and Fundamentals Governing the Design and Operation of Local Exhaust Systems, ANSI Z92-1971.60
Ventilation equipment should be c