75D - PH- MAIN PLACE TRANSFORMATION

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is estimated to increase existing traffic noise levels in the area by less than 1 dB. Because a 1 dB increase is well below the 3 dB threshold of a perceptible increase, the impact of the project -related traffic noise in the area is considered less than significant and no mitigation is required. Air Quality Environmental Setting Air Quality Standards and Monitoring Data Carbon monoxide (CO) levels are a public health concern when CO combines with hemoglobin and can reduce the rate at which oxygen is transported in the blood stream. Both the cardiovascular system and the central nervous system can be affected. State and federal ambient air quality standards for CO have been set at levels intended to keep CO from combining with more than 15% of the blood's hemoglobin. State and federal CO standards have been set for both 1-hour and 8-hour averaging times. The state 1-hour CO standard is 20 ppm, and the federal 1-hour CO standard is 35 ppm. State and federal standards are both 9 ppm for an 8-hour averaging period. State CO standards are phrased as values not to be exceeded. Federal CO standards are established as values not to be exceeded more than once per year. Both the state and federal 8-hour CO standards, as well as the state 1-hour CO standard, have been violated several times during the last four years in the project region (ARB 1994, 1993, 1992, 1991). Ozone is a public health concern because it is a respiratory irritant that also increases susceptibility to respiratory infections. Ozone causes substantial damage to leaf tissues of crops and natural vegetation, and damages many materials by acting as a chemical oxidizing agent. Ozone is formed by the combination of reactive organic gases (ROG) and oxides of nitrogen (NOx) in the presence of sunlight. State and federal standards for ozone have been set for a 1-hour averaging time. The state 1-hour ozone standard is 0.09 parts per million (ppm), not to be exceeded at any time. The federal 1-hour ozone standard is 0.12 ppm, not to be exceeded more than three times in any 3-year period. Both the state and federal ozone standards have been violated several times during the last four years in the project region (ARB 1994, 1993, 1992, 1991). Health concerns associated with suspended particles focus on those particles small enough to reach the lungs when inhaled because they can lodge in the lungs and contribute to respiratory problems, including permanent lung damage. Fine particles can also interfere with the body's mechanism for clearing the respiratory tract or by acting as a carrier of an absorbed toxic substance. Few particles larger than 10 microns in diameter reach the lungs, so particulate matter smaller than 10 microns in diameter (PM10) is the focus of the state and federal standards. State and federal PM10 standards have been set for 24-hour and annual averaging times. 'The state 24-hour PM10 standard equals 50 micrograms per cubic meter (µg/m') and the federal 24-hour standard is 150 µg/m'. The state annual PM10 standard is 30 µg/m , an annual geometric mean, whereas the federal annual PM10 standard is 50 µg/m', an annual arithmetic mean. Federal and state 24-hour PM10 Page 18 Projea No. 96.066 75D-409