PRINCIPLES OF TOXICOLOGY

fundamental viability of the work process. In some cases, however, an outright ban of the final product
may be warranted. In other words, some processes may seem so dangerous that they cannot be used
safely even with extensive control measures in place. The United States’ experiences with asbestos
(now banned from nearly all commercial use), nuclear power plants (no new power plants have been
ordered for a number of years), and chlorofluorocarbons (CFCs, which destroy the protective layer of
ozone in the upper atmosphere) are all cases where current control technologies were perceived to be
inadequate to permit further use.
In practice, the family of control measures has been relatively well described and proved. They
have come to be known as the “hierarchy of controls,” because some are more preferable than others.
The controls are listed below in order of desirability, although in practice, a combination of control
techniques is often used.
1. Substitution
2. Process modification
3. Source isolation
4. Worker isolation
5. Local exhaust ventilation
6. Dilution ventilation
7. Work practice modification
8. Administrative controls
9. Personal protective equipment (e.g., respirators)
22.3 PROGRAM MANAGEMENT 537
Substitution and Process Modification Substituting a less toxic substance for one that is more toxic,
or perhaps altogether doing away with the need to use a toxic substance, is the best of all possible
solutions. The degree of hazard is reduced or eliminated, and further controls may not be needed.
Examples of this type of control include:
• The use of water-based fountain solutions instead of isopropyl alcohol for printing operations
• The substitution of nonleaded gasoline for leaded gasoline
• The development of more efficient polymerization reactions to reduce (and practically
eliminate) the offgassing of vinyl chloride monomer in the production of polyvinyl chloride
plastic
• The substitution of cellulose and bimetallic compounds for former asbestos insulation and
brake applications
• The use of toluene or xylene instead of benzene in certain solvent applications
One note of caution: Substitution may result in simply exchanging a known hazard for an unknown
one. For example, some firms have substituted glutaraldehyde and some quaternary ammonia compounds
for formaldehyde disinfection applications. However, the substitutes have not been well
researched. As of this writing, no chronic toxicity testing has been done for glutaraldehyde, and only
subacute testing has been completed for the quaternary ammonia compounds.
Process modification may range from the simple to the complex. One manufacturer used a
chlorinated solvent to wipe grease from newly extruded plastic buckets. By adopting a more rigorous
preventive maintenance program to correct grease and oil leaks when they first appeared, the cleaning
step became largely unnecessary. A refrigerator manufacturer flame-sealed the copper tubing of the
closed-loop refrigerant system after the CFC was added. The flame generated CFC decomposition
products such as phosgene and hydrogen chloride. The process was modified so that the refrigerant
was added after the system was flame-sealed, and exposure was eliminated. On the complex side, the