CHAPTER 5 EVALUATING EXPOSURES
General Overview Exposure and Risk - What consequences do the different substances have on the environment and people? Examples of Some Regulations - Are there regulations concerning the different substances? Safer Chemical Design
Types of Exposure OCCUPATIONAL : Worker exposure in the industry. COMMUNITY : Population exposure in the industry’s surrounding area due to the waste streams.
Occupational and Community Exposure Three steps related to exposures: 1) Recognition : all sources and potential sources 2) Evaluation : level and duration of exposure 3) Control (and Elimination) : based on source, pathway, and worker/population exposure information
Occupational Exposure Recognition • Uses schematic and written descriptions to indentify : • - Potential sources of exposure (i.e. specific unit operations). • - Mechanisms that reduce worker exposure • (i.e. ventilation systems). • Exposure pathways • - Inhalation • - Dermal contact • - Ingestion
Occupational Exposure Evaluation • Monitoring worker exposure objetives include: • - Baseline • - Diagnostic • - Compliance • - Types of Monitoring : • - Personal (i.e. Breathing zone measurement) • - Area ( i.e. General monitoring to control long-term exposures) • Evaluation of occupational exposure include : • Inhalation assessment and Dermal assessment
Evaluation: Inhalation Assessment Monitoring Techniques Include : - Breathing Simulator - Static Sampler Controlling Techniques Include: - Respirators and other devices - Alternate process or modifications to equipment Models used in place of monitoring to assess inhalation are : Mass Balance Modeland Dispersion Model.
Mass Balance or Box ModelContaminant is dispersed evenly in the area (box) (5.1) • Where: • C : concentration of airborne contaminant in the work area (mass/length3), • V : volume of the work area (length3), • T : time during which the contaminant has been emitted, • G : emission rate of the contaminant to the air (mass/time), • Q : ventilation rate in the work area (length3/time), • k:a mixing factor to account for incomplete mixing in the work area (unitless), • Co : concentration of the airborne contaminant entering the work area (mass/length3).
Mass Balance or Box Model (continued) At Steady State (ss) equation 5.1 becomes the following : • (5.2) With constant ventilation a new contamination source can be estimated using : • (5.3) Where : • (5.4)
Dispersion ModelVariation of the concentration (from the source) in a given area • Where: • U is the wind velocity in the x direction (length/time) • C is the concentration of airborne contaminant (mass/length3) • D is the diffusion coefficient (lenght2/time) • xisthe distance downwind from the source (length) • r is the distance from the source to the sampling point (length) • G is the contaminant emission rate from the source (mass/time) (5.5) (5.6)
Evaluation: Dermal Exposure Assessment • The mechanisms of dermal exposure are : • - Direct contact between skin and substance. • - Transfer of substance from contaminated surface to skin. • - Deposition or impaction onto skin. • Monitoring techniques include : • - Absorbent Pads. • - Wipe Samples. • - Computerized Techniques . • Controlling techniques include : • - Wearing protective clothing and aparel. • - Substitution of a less toxic chemical (that will not impact ingestion or inhalation).
Dermal Exposure AssessmentModeling (5.7) • Where : • DAR : dermal absorbed dose rate of the chemical (mass/time), • S : surface area of the skin contacted by the chemical (length2), • Q : quantity deposited on the skin per event (mass/length2/event), • N : number of exposure events per day (event/time), • WF : weight fraction of the chemical of concern in the mixture (dimensionless), • ABS : fraction of the applied dose absorbed during the event (dimensionless).
Dermal Exposure Assessment : Modeling (continued) (5.8) • Where : • DA : dermal absorbed dose of the chemical (mass), • S : surface area of the skin contacted by the chemical (lenght2), • Kp : permeability coefficient for the chemical of concern in the mixture (length/time), • ED : exposure duration (time), • WF : weight fraction of the chemical of concern in the mixture (dimensionless), • : density of the mixture (mass/lenght3).
Community ExposureRecognition • Air Contaminants Recognition : • - Main substances and by-products that can cause harm. • - Main weather patterns and communities potentially affected by discharges. • - Phase changes (into water stream or land).
Community Exposure : Recognition (continued) • Water Contaminants Recognition : • - Main substances and by-products that can cause harm. • - Water flows and stream uses (water treatment plant, fishing, etc). • Phase changes (volatilization, ab/adsorption into solid particles, etc). • Solid Contaminants Recognition : • - Main substances and by-products that can cause harm. • - Potential leachate and volatilization of substances.
Community Exposure Evaluation • Air Exposures • - What chemicals (toxic or harmful substances). • - What quantities and from where (area, point, mobile). • - Estimate concentration in specific location (exposure location). • Dispersion models include Gaussian models (based on many factors). • - Estimate the number of people affected by contamination . • Dermal Exposures • -Frequency and duration of potential exposure (swimming only). • - Concentration of given substance.
Community Exposure : Evaluation (continued) • Surface Water • - What quantity of a given toxin remains after the wastewater. treatment process and the actual concentration in the given stream. • - Analyze the fate of the given substance using models. • - What impact do the contaminants have on aquatic organisms. • Ground Water Contamination • - Occurs from leachates (landfills) and rainwater runoffs. • - Can be transported for long distances (and into different phases) and last for long periods of time.
Persistant, Bioaccumulating and Toxic (PBT) Substances The top 12 • Aldrin/Dieldrin, • Benzo(a)pyrene, • Chlordane, • DDT • Hexachlorobenzene, • Alkyl-lead, • Mercury and Compounds, • Mirex, • Octachlorostyrene, • PCBs, • Dioxins and Furans, and • Toxaphene. References : • Binational Toxics Strategy : • http://www.epa.gov/bns/index.html • Environment Canada’s ARET program • http://www.ec.gc.ca/nopp/aret/en/el2.cfm • EPA’s PBT Chemical Program • http://www.epa.gov/pbt/index.htm
Air Pollution in the Workplace • References • OSHA : regulations of emissions in workplace • http://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=FEDERAL_REGISTER&p_id=13306 • CCOSH : general website • http://www.ccohs.ca/
Example of Emission Standards • Water and Wastewater : • - Effluent guidelines : • On a continuous basis : pH between 6.0 and 9.5 • On a monthly average basis : Source : http://www.ec.gc.ca/nopp/docs/cp/1mm8/en/c4.cfm
Safer Chemical Design • Key goals of designing safer chemicals areminimizing : • - Persistence and Dispersion in the environment (and therefore reducing exposure). • - Bioaccumulation and reducing dose (thereby reducing the uptake by the body). • - Toxicity. • Safer Chemical Design include : • - Dose minimization. • - Toxicity minimization.
Safer Chemical Design Dose Reduction • Information needed to calculate doses: • - Mass of the chemical transfered across a certain membrane. • - Depending of the different membranes, chemical and physical properties are needed: • Lung : water solubility, particle size. • Gastrointestinal tract : lipid solubility, water solubility, dissociation constant and molecular size. • Skin : lipid solubility.
The lung also provides a relatively large surface area for uptake of chemicals. The lung is a relatively thin membrane and because the membrane is so thin, lipid solubility plays less of a role in chemical uptake than for the gastrointestinal tract. High water solubility will promote uptake through the lung, as will the delivery of the compound on fine particles.
The skin presents a formidable barrier to chemicals transport. For a chemical to be taken up through the skin, it must pass through multiple layers. As with the gastrointestinal tract, moderate lipophilicity promotes absorption through the skin because transport must occur through both largely lipid and largely aqueous layers. High water solubility enhances uptake trough the gastrointestinal tract because water soluble materials are more easily mobilized in the large and small intestine and the materials therefore experience less mass transfer resistance in migrating to the intestine wall. High lipid solubility enhances uptake and transport across the membrane.
Safer Chemical Design Toxicity Reduction • Important information is obtained by : • - Examining mechanisms. • - Identifying structural mechanisms.
Toxicity Reduction Evaluations (TREs) TREs use toxicity tests, detailed chemical analyses, and process evaluations to determine the cause of effluent toxicity. These evaluations explore treatment options to reduce toxicity to acceptable levels or identify changes within a facility to alter the type, quantity, or character of the discharge. We then identify the type and source of toxins and then make an evaluation of treatment alternatives. When the TRE is complete, we prepare a final report which contains recommendations for toxicity reduction or elimination that will bring a facility back into compliance.