Estimating Risks from Vapors

The TLV provides a simple means to evaluate the relative risk of exposure to the vapor of any substance used in the laboratory. If the quantity of the material evaporated is represented by m (in milligrams/hour) and the TLV is expressed by L (milligrams per cubic meter), a measure of relative risk to the vapor is given by m/L. This quantity represents the volume of clean air required to dilute the emissions to the TLV. As an example, the emission of 1 g of bromine and 10 g of acetone in one hour leads to the values of m/L of 1400 m /hour (h) for the bromine and 5.6 m /h for acetone.These numbers provide a direct handle on the relative risks from these two vapors. It is difficult to assess the absolute risk to these vapors without a lot of information about the ventilation characteristics of the laboratory. If these releases occur within a properly operated hood, the threat to the worker in the laboratory is probably very small. (However, consideration must be given to the hood exhaust.) 

Exposure in the general laboratory environment can be assessed if we assume that the emissions are reasonably weU mixed before they are inhaled and if we know something about the room ventilation rate. The ventilation rate of the room can be measured by a number of ways. Given the ventilation rate, it might be safe to assume that only 30% of that air is available for diluting the emissions. (This accounts for imperfect mixing in the room.) The effective amount of air available for dilution can then be compared with the amount of air required to dilute the chemical to the TLV. 

Let us continue our example. Suppose that the laboratory has a volume of 75 m and an air exchange rate of 2 air changes per hour. This value means that (75 m )(2/h)(0.3) = 45 m /h are available to dilute the pollutants. There may be enough margin for error to reduce the acetone concentration to a low level (5.6 m /h is required to reach the TLV), but use of bromine should be restricted to the hood. An assessment of the accumulative risk of several chemicals is obtained by adding the individual m/L (j Jfe) values. 

The m/L figures may also be used to assess the relative risk of performing the experiment outside a hood. Since m/L represents the volume of air for each student, this may be compared with the volume of air actually available for each student. If the ventilation rate for the entire laboratory is Q (in cubic meters per minute) for a section of n students meeting for t minutes, the volume for each student is kQt/n cubic meters. Here fc is a mixing factor that allows for the fact that the ventilation air will not be perfectly mixed in the laboratory before it is exhausted. In a reasonable worst-case mixing situation a k value of 0.3 seems reasonable. Laboratories with modest ventilation rates supplied by 

15-20 linear feet of hoods can be expected to provide 30-100 m per student over a 3-h laboratory period if the hoods are working properly. Let us take the figure of 50 per student as an illustration. If the value of m/L for a compound (or a group of compounds in a reaction) is substantially less than 50 m, it may be safe to do that series of operations in the open laboratory. If m/L is comparable to or greater than 50 m, a number of options are available (1) Steps using that compound may be restricted to a hood. (2) The instructional staff may satisfy themselves that much less than the assumed value is actually evaporated under conditions present in the laboratory. (3) The number of individual repetitions of this experiment may be reduced. The size of the laboratory section can be reduced or the experiment may be done in pairs or trios. 

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EPA. 1984a. Estimation of the public health risk from exposure to gasoline vapor via the gasoline marketing system. Washington, DC U.S. Environmental Protection Agency. 

Figure 1 shows part of a solvent phase polypropylene plant. The plant consists of three process lines, denoted A, B, and C. During a risk assessment review, a scenario was identified that involved a release of reactor contents from a location near the west end of the A line. Estimates are needed of the blast overpressures that would occur if the resulting cloud of vapor, mist, and power ignites. 

Environmental tobacco smoke mid gasoline vapors both contain mixtures of trace luiiounts of many of the individual compounds regulated as Air Toxics under Title 111, section 112 of the 1990 Clean Air Act Amendnmts. Much of the general public is more likely to be exposed to these mixtures during the course of their lives tlian to specific compounds on the air toxics list. Hence, estimation of the cancer risk resulting from exposure to these mixtures is a useful and relevant exercise. 

In 1989 the Environmental Protection Agency ordered a 90% reduction of industrial benzene emissions over the next several years at a cost of 1 billion. The new standard leaves more than 99% of the exposed population with risks of cancer less than one in 1 million, or one cancer case in the U.S. every 10 years. Hardest hit are the iron and steel industry, where benzene emissions from coke by-product recovery plants are large. Chemical industry plants have already reduced their benzene emissions 98%. EPA estimates that the 390,000 or so gasoline service stations in the U.S. will all have to be fitted with devices to eliminate the escape vapors when fuel is put into underground storage tanks. 

Halothane (boiling point [BP] 50°C), en-flwane (BP 56°C), isoflurane (BP 48°C) and the newer substances, desflurane and sevo-flurane, have to be vaporized by special devices. Part of the administered halothane (up to 20%) is converted into hepatotoxic metabolites (B). Liver damage may result from halothane anesthesia. With a single exposure, the risk involved is unpredictable however, the risk increases with the frequency of exposure and the shortness of the interval between successive exposures (estimated incidence 1 in 35 000 procedures). 

Considering all the above data, the U.S. EPA (1991) selected the unit risk of 8.5 x 10 per pg/m, derived from the Weibull time-to-tumor model, as the recommended upper bound estimate of the carcinogenic potency of sulfur mustard for a lifetime exposure to HD vapors. However, U.S. EPA (1991) stated that "depending on the unknown true shape of the dose-response curve at low doses, actual risks may be anywhere from this upper bound down to zero". The Weibull model was considered to be the most suitable because the exposures used were long-term, the effect of killing the test animals before a full lifetime was adjusted for, and the sample size was the largest obtainable from the McNamara et al. (1975) data. 

Exposure to indoor hydrocarbon vapors can occur as a consequence of changes of land use from commercial or industrial to residential. Residual hydrocarbons in soils or groundwater may result in a chronic vapor exposure pathway. Analysis of risk associated with exposure to hydrocarbons typically is undertaken in a multi-step approach known as risk assessment. A thorough discussion of the use of risk assessment at contaminated sites is provided in Chapter 9.01. Assessing risks posed by hydrocarbon spills or wastes is complex and involves estimates of chemical concentrations at each potential exposure point, identification of the potential populations that may be exposed, and assessment of exposure pathways, intake rates, and the toxicity of the chemicals of concern. 

In the approximately 70 years since the discovery of the toxic G agents and 50 years since the subsequent development of the V agents, humans have only occassionally served as test subjects in laboratory studies designed to determine threshold toxic effects associated with low-level (nonlethal) sarin and VX vapor exposures (2-10 min) (Johns, 1952 Sim, 1956 Bramwell et al., 1963). In addition, although the toxic effects of accidental exposures and nonexperimental exposures from terrorist or military attacks are documented, critical information related to the exposure conditions can only be estimated at best. Thus, estimates of human dose-responses to nerve agent vapor exposures from such sources are often associated with significant uncertainty and are of limited utility in predicting health hazard risks. 

Dichlorobenzene (DCB) in mothballs provides a different type of pesticide example. A cancer bioassay conducted by NTP (1987) found excess tumors in both rats and mice. The Intemational Agency for Research on Cancer (lARC) concluded that DCB is reasonably anticipated to be a human carcinogen, and it is listed by the State of California as a chemical known to cause cancer (see below). DCB in mothballs results in low vapor levels in areas where it is used, and vapors emanate from clothes worn after storage with DCB. An estimated human inhalation cancer risk value is available (OEHHA, 2005). Despite the widespread exposure and recognized cancer risk, there are no restrictions on the use of DCB for this purpose. 

When the commander directs his unit to go from MOPP Zero to M0PP1, the chemical officer/NCO will determine BDO/CPOG days of wear. Upon completion of the mission, the unit returns to MOPP Zero, and overgarments are returned to their vapor-barrier bag. The chemical officer/NCO at company or battalion level estimate the number of days of BDO/CPOG wear for his unit. Then uses the information on days of overgarment wear as input for risk assessment. 

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