Chapter 13. Assessment Of Health Effects
13.1. Assessment Of Health Effects from Chemical Releases
13.2. Assessing Chemical Impact
13.3. Chemical Characterization
13.4. Chemical Characterization(contd.)
13.5. Chemical Characterization(contd.)
13.6. Chemical Characterization(contd.)
13.7. Assessing a Potential Release Magnitude
13.8. Assessing a Potential Release Toxicity
13.9. Assessing a Potential Release Toxicity(contd.)
13.10. Assessing a Potential Release Toxicity(contd.)
13.11. Assessing a Potential Release Exposure
13.12. Assessing a Potential Release Health Risk
13.13. Assessing a Potential Release Toxic Air Emissions
13.14. Assessing a Potential Release Toxic Air Emissions(contd.)
13.15. Assessing a Potential Release Toxic Air Emissions(contd.)
13.16. Assessing a Potential Release Toxic Air Emissions(contd.)
13.1. Assessment Of Health Effects from Chemical Releases
Chemicals released into the environment carry a potential risk to human health. The extent of risk depends on many variables. These include the chemical and exposure from the release.
Exposure can either be chronic or acute. Chronic exposure occurs in small amounts of the chemical over an extended period of time. Acute is exposure to a high concentration in a short time period.
This section will emphasis acute exposure from a sudden accidental chemical release. Assessing health effects from a potential release is a vital part of performing a risk assessment.
13.2. Assessing Chemical Impact
There are five necessary steps for assessing the impact of a chemical on human health. These are listed on the slide and include the characteristics and toxicity of the chemical, the magnitude of potential release and the potential exposure. These steps will be described on the next slides.
13.3. Chemical Characterization
Chemical characterization involves determining the physical characteristics of the chemical(s) involved in a potential release. This includes the physical state: solid, liquid or gas. Other important physical properties for the assessment include vapor pressure, density, solubility, melting and boiling point, reactivity or stability, ionization potential and flammability as measured by the upper and lower explosion limits and flash point. These are discussed briefly on the following slides.
13.4. Chemical Characterization (continued)
Vapor Density:
- Density of the chemical vapor relative to the density of air at the same temperature
Solubility:
- Percentage of solute, by weight, that will dissolve in water at a given temperature.
-Must consider solubility because of ability of released particulate that may settle and contaminate surface water
Vapor density is the density of the chemical vapor relative to the density of air at the same temperature. Chemicals with a density less than air will rise and tend to be dispersed while those with a density greater than air will tend to collect on the ground and low spots. Solubility in water will give an indication of transport in rivers and ground water or contaminate surface water.
13.5. Chemical Characterization (continued)
When the vapor pressure of the liquid equals the atmospheric pressure
Describes the stability of a substance as well as its compatibility with other substances, its decomposition products, and likelihood of polymerization
Releases of chemical with boiling points above ambient temperature lead to vapor clouds and those below ambient lead to pools of liquids. Reactivity and stability data are used to determine the compatibility with container materials, for example. It also describes potential decomposition from sensitivity to light, moisture, shock and the likelihood of polymerization
13.6. Chemical Characterization (continued)
Ionization potential is the amount of energy needed to remove an electron (ionize) the compound. This characteristic is used in monitoring equipment such as photo ionization detectors or flame ionization detectors.
The flash point is the lowest temperature at which a liquid will produce enough vapor to create an ignitable mixture in air (at standard pressure), and this determines safeguards from equipment with switches that are not sealed, for example.
13.7. Assessing a Potential Release Magnitude
Magnitude:
The magnitude of a release is usually given in terms of the amount of the release in pounds per minute and time. It also refers to the surrounding population that is affected, atmospheric conditions that may affect dispersion, and size of the affected area.
13.8. Assessing a Potential Release Toxicity
Toxicity: -
Determining toxicity is the third step required to assess health effects from chemical releases. As shown in the slide, toxicity refers to the ability of a substance to adversely affect the health of an organism. The higher the toxicity of a substance, the more hazardous it is. Toxicity is affected by the route of exposure e.g., inhalation, ingestion, or skin contact. Additional information is given on the next slide.
13.9. Assessing a Potential Release Toxicity (continued)
- IDLH represents the maximum concentration a worker can be exposed to without permanent adverse health effects
- the amount that kills 50 % of the exposed test population
- The concentration that kills 50 % of the exposed test population through inhalation
Toxicity may be measured in many ways. These include the ones listed on this slide. Immediately dangerous to life and health (IDLH) is the maximum concentration a worker can be exposed to without permanent adverse health effects. The median lethal dose (LD50) is the amount of chemical that kills 50% of the exposed test population. The median lethal concentration (LC50) is the concentration of chemical that kills 50% of the exposed test population through inhalation. Other important measures are on the next slide.
3.10. Assessing a Potential Release Toxicity (continued)
Refers to the lowest confirmed concentration level that caused death or permanent disability to the test subject
Gives a level of contaminant to which an average healthy worker can be exposed for 8 hours a day, 5 days a week, without experiencing adverse health effects
The use of PEL was illustrated in the section on Screening Methods to evaluate when a room was safe to enter after ventilating from a release of a toxic material.
The use of PEL was illustrated in the section on Screening Methods to evaluate when a room was safe to enter after ventilating from a release of a toxic material.
13.11. Assessing a Potential Release Exposure
Exposures are classified as either acute or chronic. Chronic exposure is the exposure to low concentrations of a toxic material over a long period of time, for example months or years. Acute exposure is the exposure to a high concentration of a toxic material for a short interval on the order of seconds or minutes. The Level of Concern (LOC) is a measure used to evaluate health risks and is the concentration of an extremely hazardous substance in the air where there may be serious irreversible health effects.
13.12. Assessing a Potential Release Health Risk
Inhalation dose estimate IEX (mg/kg-day)
IEX = DICF/B(25,600/lifetime)
The final step in the health effects assessment process is to estimate the health effects experienced by the surrounding population. The level of concern LOC is the concentration of an extremely hazardous substance in the air where there may be serious irreversible health effects. When this concentration is reached EPA recommends that the area should be targeted for evacuation. The dose received by an individual in a exposed area can be estimated by the equation given on this slide.
13.13. Assessing a Potential Release Toxic Air Emissions
For an accidental release of a toxic chemical to the atmosphere, it is important to estimate the concentrations of the chemical at various locations at any time in the future. Chemical plume dispersion analysis is used to estimate this concentration distribution.
For an accidental release of a toxic chemical to the atmosphere, it is important to estimate the concentrations of the chemical at various locations at any time in the future. Chemical plume dispersion analysis is used to estimate this concentration distribution. This procedure is illustrated with an example on the next slide.
13.14. Assessing a Potential Release Toxic Air Emissions (continued)
A pressure vessel containing 1500 lbs of hydrogen sulfide suddenly ruptures, releasing the contents to the atmosphere. The chemical plume dispersion analysis is to be used to estimate the concentration distribution for the greatest distance downwind from the release at which the H2S concentration exceeds the threshold limit value, 10 ppm. This concentration is the level at which there is imminent danger to life or health (the “IDLH” level)
13.15. Assessing a Potential Release Toxic Air Emissions (continued)
The ambient temperature is 70oF, and the plant is at sea level. The weather is bright and sunny. There is a 10 mile per hour wind from the north. The molecular weight of H2S is 34.08.
The equation for chemical plume dispersion for estimating the ground level, downwind concentration from an instantaneous release of a vapor is:
The dispersion coefficients are functions of weather conditions and distance from the source. Under the conditions of this problem the dispersion coefficients were correlated by:
σx = σy = 0.16 x 0.92 and σz = 0.71 x 0.73, with the σ’s and x in feet
This slide gives the conditions at the plant at the time of the release. Also, the equation for the chemical plume dispersion is given. It has the conditions added for this location and atmospheric conditions. This Gaussian plume equation was described in the section on Chemical Plume Dispersion Analysis. Use this information to determine the distance from the source where the concentration is greater than 10 ppm. Compare your results with the evaluation on the next slide. Note that the maximum concentration C is when x = ut in the dispersion equation.
13.16. Assessing a Potential Release Toxic Air Emissions (continued)
Calculate the lb moles of H2S released:
M = (1500)/(34.08) = 44.01 lb moles
This converts lb to lb moles.
Calculate the molar concentration of air at these conditions:
Cair = P/RT = (14.7)/(10.73)(530) = 0.00258 lb moles/ ft3
Use the ideal gas law to evaluate the lb moles of air per ft3.
Calculate the threshold limit concentration, of H2S:
Ct = 10-5 Cair = 2.58x10-8 lb moles/ft3
The threshold limit is 10 ppm molar Ct, basis or 10-5 times the air concentration.
Inserting this information in the equation for chemical plume dispersion and recognizing that the maximum value for C at any point occurs when x = ut. Using Ct = 2.58x10-8 lb moles/ft3:
Ct = 2.58x10-8 lb moles/ft3 = [(2) (44.01)]/[(2 )1.5(0.16 x0.92 )(0.71 x0.73 )]
Solving for x gives:
x = 8,940 ft = 1.69 miles
This is the distance that the H2S concentration is greater than or equal to 10 ppm.
To calculate the molar concentration of air at these conditions, the ideal gas law is used to evaluate the air concentration as shown on the slide. Then the threshold limit concentration of H2S of 10 ppm is expressed as a concentration in lb moles/ft3. Then this information is inserted in the plume equation as shown on the slide, and the distance x is evaluated as 1.69 miles.