A Closer Look at Air Pollution in Houston: - Green Houston

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al average concentrations for 1999 from EPA's National-scaleAir Toxics Assessment (NATA) (U.S. EPA, 2006d). A descriptionof NATA 1999 is presented in Appendix 2. Results fromthe NATA provided estimated ambient concentrations for 177substances (176 HAPs and diesel particulate matter) in 895census tracts (each with approximately 4,000 inhabitants)included in the 10-county Greater Houston area. The NATAvalues were derived by EPA using a computerized air dispersionmodel that combined 1999 airborne emissions data fromoutdoor sources, including point,mobile (on-road and non-road),area, and background sourceswith Houston-specific meteorologicalvariables. The model also tookinto consideration the breakdown,deposition and transformation ofpollutants in the atmosphere aftertheir release. The Task Force supplementedthese data with measured2004 annual concentrationsfor 50 pollutants (49 HAPs plus adiesel particulate matter surrogate)from 20 monitoring sites inand around Houston - 14 in HarrisCounty, 4 in Galveston, 1 inBrazoria, and 1 in Montgomery.These data were obtained fromEPA's Air Quality System (AQS);for a description of the AQSdataset see Appendix 2. AllAQS data used for risk rankingPhoto by Heidi Bethelwas from 2004 (U.S. EPA, 2006e), the most recent year forwhich complete data were available.To get a sense of relative health risks associated with estimatedambient concentrations of HAPs, the Task Force usedhealth-related toxicity values developed for health risk assessmentsby either the U.S. EPA or the California Office ofEnvironmental Health Hazard Assessment (OEHHA), whichevervalue was the more stringent (health protective) (CaliforniaEPA & OEHHA, 2002; California OEHHA, 2005; U.S. EPA, 2005,2006h, 2006i). In instances when no value was developed byUS EPA or California OEHHA, health values from other availablesources were used. A detailed table of health values is presentedin Appendix 3, Table A3.1. For carcinogens, estimateswere based on their respective unit risk values (UREs),which represent the excess lifetime cancer risk estimated toresult from continuous lifetime exposure to an average concentrationof 1 microgram per cubic meter (µg/m3) of a certain pollutantin the air. For noncarcinogens, estimates were based oncomparison of estimated ambient concentrations with theirrespective chronic non-cancer inhalation health values: referenceconcentrations (RfC) - usedby U.S. EPA; reference exposureslevels (REL) - used by CaliforniaOEHHA; or minimum risk levels(MRL) - used by the Agency forToxic Substances and DiseaseRegistry (ATSDR).Each HAP was assigned initiallyto a specific risk categorycontingent on how measured ormodeled annual-average concentrationstranslated into comparativerisk estimates using establishedUREs (carcinogens) and/orRfCs, RELs, or MRLs (noncarcinogens).Initial risk-category assignmentswere adjusted, as necessary,based on evaluation of additionalinformation about relativeemission quantities and number ofcensus tracts or monitoring stationsaffected. See Appendix 1for a thorough explanation on the ranking process.Final Risk Categories - Using the process outlinedabove, the Task Force assigned each of the 179 air pollutants(176 HAPs modeled and/or monitored, ozone, fine particulatematter, and diesel particulate matter) to one of five comparativerisk categories. Substances were designated “UnlikelyRisks” when there was suggestive evidence of negligible orinsignificant risk to the general population and vulnerable subgroups.Substances were deemed “Uncertain Risks” whenthere was inadequate or insufficient evidence to ascertainwhether they posed a significant risk to the general population12
and vulnerable subgroups. Substances were designated“Possible Risks” when there was partial or limited evidencethat suggested they might constitute a significant risk undercertain circumstances, and “Probable Risks” when therewas substantial corroborating evidence that they were likely torepresent a significant risk under the right conditions. Thosesubstances for which there was compelling and convincing evidenceof significant risk to the general population or vulnerablesubgroups at current ambient concentrations were labeled“Definite Risks.”As shown in Table 1, 12 air pollutants were classified as“Definite Risks”. The Task Force found that existing and projectedambient concentrations of two criteria pollutants - ozoneand fine particles (PM 2.5) - are almost certainly causing respiratoryand cardiopulmonary effects in some individuals as wellas contributing to premature death. It was also determined thatairborne concentrations of seven carcinogens - diesel particulatematter (see Appendix 4 for more detail on this pollutant),1,3-butadiene, chromium VI (see Appendix 4 for more detailon this pollutant), benzene, ethylene dibromide, formaldehyde,and acrylonitrile - pose an unacceptable increased cancer risk.In addition, it was concluded that five substances -- 1,3-butadience(reproductive effects in addition to being a carcinogen),formaldehyde (respiratory effects), acrolein (respiratoryeffects), chlorine (respiratory effects), hexamethylene diisocyanate(pulmonary and respiratory effects). -- are present atambient concentrations that represent an unacceptableincreased risk for chronic disease in Houston.The evidence is not as strong but nevertheless persuasivethat an additional 9 air pollutants are likely to pose unacceptablehealth risks at concentrations measured or modeledin Houston air. These substances were designated as“Probable Risks,” and included eight carcinogens - vinyl chloride,acetaldehyde, ethylene dichloride, naphthalene, arseniccompounds, carbon tetrachloride, ethylene oxide, 1,1,2,2-tetrachloroethane- and one pollutant - acrylic acid - that has chronicnon-cancer effects. These are shown in Table 2 on pg. 14.13
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Page 3 and 4: A Closer Look at Air Pollution in H
Page 5 and 6: Task Force MembersResearch StaffKen
Page 7 and 8: AcknowledgementsThe Task Force woul
Page 10: Just because a task is difficult, h
Page 13: ing, and route of exposure, and oth
Page 17 and 18: accurately from death certificates.
Page 19 and 20: assessments, and monitoring data fr
Page 21 and 22: The reality is that, even at simila
Page 23 and 24: ability, have inadequate means to c
Page 25 and 26: Houston reaches $21,701. These are
Page 27 and 28: In summary, we view the comparative
Page 29 and 30: AppendicesPhoto by Heidi Bethel27
Page 31 and 32: Photo by Heidi Bethelgrouping to a
Page 33 and 34: Appendix 2the time period is repres
Page 35 and 36: Appendix 3an appreciable risk of ad
Page 37 and 38: Table A3.1. Health Values and Refer
Page 43 and 44: Appendix 4Metals, Diesel PM Convers
Page 45 and 46: Under advice from staff at the U.S.
Page 47 and 48: Appendix 5Table A5.1. Uncertain Ris
Page 49 and 50: Appendix 6Brief Descriptions of Hea
Page 51 and 52: Appendix 7Summary Map and Table for
Page 53 and 54: Appendix 8: Table 1Supplemental Tab
Page 55 and 56: Appendix 8: Table 3Table A8.3. Poss
Page 57 and 58: ReferencesHackett, P., Sikov, M., &

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