PRINCIPLES OF TOXICOLOGY

538 CONTROLLING OCCUPATIONAL AND ENVIRONMENTAL HEALTH HAZARDS
U.S. Air Force has been conducting research on the use of robotic lasers to strip paint from its aircraft
to replace the current method of using methylene chloride–based chemical strippers.
In the late 1990s, some legislation was passed at the state level requiring industry to reduce the use
of toxic substances on a certain schedule. Whether these targets are realistic remains to be seen.
However, these initiatives could radically alter the prevailing conception of what is and is not feasible
in the development of new processes and new substitutes.
Source Isolation Source isolation can effectively prevent exposure, and thus reduce the hazard. A
potential hazard still remains, however, since a leak can develop and result in exposure. The Union
Carbide incident at Bhopal, India in 1993 may be the best-known example. Backup alarm systems
warning of a breech in the isolating mechanism are usually specified for this type of control measure,
and emergency response procedures must be established. Specific requirements for process safety
management for processes which use highly hazardous chemicals have been established by OSHA.
Enclosures with their own dedicated exhaust ventilation systems are another type of source
enclosure. In poultry hatcheries, formaldehyde is used to disinfect the eggs, which are isolated inside
an incubation chamber. Planning must include specification of those work processes that will require
entry into the enclosure, and the types of protective measures, such as the type of respirator to be used,
and how much ventilation of the enclosure will be needed after production has ceased before entry can
be safely completed. Maintenance activities and emergency-response operations often require entry
into such enclosure systems. In the case of the hatchery, at least 60 min of purge time at 20 air changes
per hour or greater must be allowed before a worker enters the booth. To provide adequate mixing
within the booth, velocities through air inlet doors should be at least 500 feet per minute (fpm), and
velocities through large access doors should be at least 100 fpm.
Worker Isolation If the toxic substance cannot be isolated, then perhaps the worker can be. For
example, it is not feasible to enclose a large railcar coal dumping station at a large power plant. However,
a small operator’s booth, with its own filtered source of fresh, tempered air, is certainly feasible. Again,
plans should include provisions for emergency exit from the enclosure into the hazardous atmosphere.
Many worker isolation enclosures fail because they do not provide the necessary comfort. If heated
or cooled fresh air is not supplied to the extent necessary, the employee is likely to open the door,
resulting in potential overexposure. These provisions are sometimes regarded as an unnecessary
“creature comfort” expense. In reality, they need to be viewed as an integral part of the control
mechanism, for without it, the control fails. Even if employees are trained in the hazards of compromising
the enclosure’s integrity, on hot summer days the worker may view the immediate problem of
baking inside the enclosure as worse than being exposed to an undetectable toxicant. This is a flaw in
the design, not “human nature.”
Local Exhaust and Dilution Ventilation If it is not feasible to substitute for the chemical, modify the
process, or prevent the release of the air contaminant into the workers’ environment, then local exhaust
or dilution ventilation will be needed. Of the two, local exhaust ventilation is more desirable, because
more complete capture of the contaminant is possible in most cases and smaller amounts of air will
need to be moved, resulting in energy cost savings.
Local exhaust ventilation systems consist of a hood, ductwork, a fan, a pollution-control device,
and an exhaust stack. Hoods can be of the external or internal variety. The external hood is designed
to capture air contaminants released some distance in front of the hood, while the internal hood controls
the contaminant inside some type of partial enclosure. Figure 22.2 shows a welding fume extractor
and a laboratory hood, illustrating the differences between the two types of hood. Generally, a greater
degree of control can be exerted by an internal hood, since the chances of cross-drafts and other sources
of turbulence are less. However, even the best designed laboratory hood will exhibit some degree of
leakage.
Training of employees in the proper use of ventilating equipment is crucial. Laboratory workers
need to understand the consequences of placing too many bottles inside their hoods or not placing the