The distance to the toxic endpoint was calculated for both the worst and alternate case release scenarios. The toxic endpoint for chlorine is 0.0087 mg/l (3 ppm). This value is based on the Emergency Response Planning Guidelines Level 2 (ERPG‐2) value. ERPG‐2 is defined as the maximum airborne concentration below which it is believed that nearly all individuals could be exposed for up to one hour without experiencing or developing irreversible or other serious health effects, or symptoms that could impair an individual’s ability to take protective action.
Highly specialized computers software was used to estimate the distance to the toxic endpoint for each scenario. The programs, RMP*Comp, version 1.06, is a mathematical tool developed by EPA and NOAA specifically to determine the endpoint distances from various releases under the RMP rule (for details, visit the U.S. government website http://response.restoration.noaa.gov/chemaids/rmp/rmp.html, http://www.epa.gov/rmp/rmpcomp; USEPA 1996).
Table 5.8 shows the input used in RMP*Comp. RMP*Comp assumes a meteorological condition of class F stability with a 1.5 m/s wind speed for worst‐case releases and, for alternate releases, a meteorological condition of class D stability and a 3 m/s wind speed. The model requires choosing between urban and rural dispersion patterns. Urban dispersion was chosen for this analysis because the site is surrounded by residential and industrial development.
Determination of Exposed Population to this Scenario
Population information was obtained using LandView III, an electronic mapping software system that includes database information from the EPA, the Bureau of Census, the U.S. Geological Survey, and other federal agencies. LandView III was developed at the EPA’s Chemical Emergency Preparedness and Prevention Office in Washington, DC. The software includes two different calculation methods for estimating the population within a radius of a given point using an updated Census data. The methodology used for this analysis was the default method, called the Block Group Proration method, which sums the data for each census block group that has any portion falling within the radius of the circle and then prorates the result based on the ratio of the area of the circle to the total land area of the block groups. The results of the calculation are summarized for this scenario in Table 5.9.
Table 5.8 RMP*Comp inputs.
| Worst‐case inputs | Alternative case inputs | |
| Release type | Vapor | Vapor |
| Amount released (lb) | 2000 | 900 |
| Release rate (lb/min) | 200 | 31.3 |
| Release duration (min) | 10 | 20 |
| Wind speed (m/s) | 1.5 | 3.0 |
| Atmospheric stability class | F | D |
| Roughness type | URBAN | URBAN |
| Passive mitigation measures | Release outside building | Release inside building |
| Active mitigation measures | None | None |
| Tank parameters | ||
| Hole area | Not considered | 1.25 × 10−4 in2 (0.15 in. diameter) |
| Tank temperature (°C) | 25 | 25 |
| Tank pressure (psi) | 120 | 120 |
Chronic Industrial Exposure: TWA and TLV
The use of the TWA, which has been adopted by OSHA, enables in assessing exposure during a eight‐hour periods of time as follows:
where
- Ca, Cb,…,Cn = concentrations of first, second, …, and final durations of exposure periods, respectively.
- Ta, Tb,…,Tn = time of first, second, …, and final time exposure periods (hours), respectively.
Table 5.9 Population estimates within endpoint circles.
| Worst case | Alternate case | |
| Distance to endpoint (miles) | 1.3 | 0.1 |
| Estimated residential population within distance to endpoint | 13 322 | 5 |
| Public receptors within distance to endpoint | Residences, commercial and industrial facilities, recreation areas, hospital | Residences, recreation areas |
| Environmental receptors within distance to endpoint | Parks | None |
Chronic toxicity in hazardous waste management is most often caused by long‐term, low‐level exposure to hazardous chemicals. Chronic toxicity is difficult to quantify, because it is less known about the long‐term effects of chemicals compared to acute toxicity evaluations. A parameter used by industrial hygienists, the TLV, is the concentration of a chemical in air to which workers may be exposed safely over a working over their occupational lifetime (i.e. for 8 h/day, 5 days/week, over a working lifetime). TLVs have been defined for hundreds of airborne contaminants. Some contaminants (e.g. carcinogens) have been given a zero TLV because no threshold exists. The TLV is a copyrighted term of the American Conference of Governmental Industrial Hygienists (ACGIH), a group that develops policy and continually updates TLV data. Table 5.10 presents TLVs of some common hazardous compounds. The TLV ceiling value is the concentration that should not be exceeded at any time (ACGIH 1986).
Table 5.10 TLVs of some common hazardous compounds.
Source: From ACGIH (1986).
| Compound | TLV |
| Acetone | 750 ppm |
| Aldrin | 0.25 mg/m3 |
| Atrazine | 5 mg/m3 |
| Benzene | 10 ppm |
| Carbaryl | 5 mg/m3 |
| Carbon tetrachloride | 5 ppm |
| Chlorobenzene | 10 ppm |
| Chloroform | 10 ppm |
| Chrysene | Suspected human carcinogen |
| Cyclohexane | 300 ppm |
| Cyclohexene | 300 ppm |
| DDT | 1 mg/m3 |
| 2,4‐D (Dichlorophenoxyacetic acid) | 0.1 mg/m3 |
| Dieldrin | 0.25 mg/m3 |
| Dimethyl phthalate | 5 mg/m3 |
| Ethylbenzene | 100 ppm |
| n‐Heptane | 400 ppm |
| Hexachlorobenzene | 0.025 mg/m3 |
| Hexachlorocyclopentadiene | 0.01 ppm |
| Malathion | 10 mg/m3 |
| Methyl butyl ketone | 50 ppm |
| Methyl ethyl ketone (MEK) | 200 ppm |
| Methyl isobutyl ketone (MIBK) | 50 ppm |
| Methylene chloride | 50 ppm |
| Naphthalene | 10 ppm |
| n‐Octane | 300 ppm |
| Parathion | 0.1 mg/m3 |
| Pentachlorophenol | 0.5 mg/m3 |
| Perchloroethylene (PCE) | 50 ppm |
| Phenol | 5 ppm |
| Picric acid | 0.1 mg/m3 |
| Polychlorinated biphenyls (PCBs) | 0.5 mg/m3 |
| Toluene | 100 ppm |
| 1,1,2‐Trichloroethane (TCA) | 10 ppm |
| Trichloroethylene (TCE) | 50 ppm |
| 2,4,6‐Trinitrotoluene (TNT) | 0.5 mg/m3 |
| o‐Xylene | 100 ppm |
| m‐Xylene | 100 ppm |
| p‐Xylene | 100 ppm |
EXAMPLE 5.11
Estimate the time weighted concentration for 8 hours if a worker is exposed to concentrations of 100 ppm for 1 hour, 90 ppm for 2 hours and 80 ppm for 5 hours.
SOLUTION
Using Eq. (5.8) we get
If more than one chemical is present in the workplace, one procedure is to assume that the effects of the toxicants are additive. The combined exposures from multiple toxicants with different TLV–TWAs are determined from the equation:
where
- n is the total number of toxicants
- Ci is the concentration of chemical i with respect to the other toxicants
- (TLV–TWA)i is the TLV–TWA for chemical species i.
If this sum in Eq. (5.9) exceeds 1, then the workers are overexposed.
The mixture TLV–TWA can be computed from
(5.10)
If the sum of the concentration of the toxicants in the mixture exceeds this amount, then the workers are overexposed.
For mixture of toxicants with different effects (such as an acid vapor mixed with lead fume) the TLVs cannot be assumed to be additive (Crowl and Louvar 2011).
EXAMPLE 5.12
Determine the mixture of TLV–TWA for a worker exposed to 42 ppm PCE, 38 ppm TCE, 86 ppm MEK, and 35 ppm MIBK in the air of a RCRA solvent recycling operation. Are the concentrations of chemicals exceeding the TLV–TWAmixture?
From Table 5.10, the TLVs for the chemicals are
| Compound | TLV (ppm) |
| PCE | 50 |
| TCE | 50 |
| MEK | 200 |
| MIBK | 50 |
Using Eq. (5.9), these values are applied to determine the TLV–TWAmixture over an 8‐hour workday.
The total exposure, ∑Ci = 42 + 38 + 86 + 35 = 201 ppm; because the ratio of ∑Ci/(TLV − TWA)mix is greater than 1, the concentrations of these RCRA chemicals exceeded the limit. The air quality in the solvent recycling facility is unhealthy based on the TLVs.
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