Aviation Emissions and Air Quality Handbook  2.2.1.  National Ambient Air Quality Standards  EPA has promulgated National Ambient Air Quality Standards ( NAAQS) 5  to safeguard public  health and environmental welfare against the detrimental effects of outdoor air Primary NAAQS are health -based standards geared toward protecting sensitive or at -risk  portions of the population such as asthmatics, children,  and the elderly. Secondary NAAQS are  welfare oriented and are designed to prevent decreased visibility and damage to vegetation, and physical structures.  NAAQS have been established for s ix criteria air pollutants shown in  Table 2-1  ( National  Ambient Air Quality Standards ) and they include carbon monoxide ( CO), lead (Pb), nitrogen  dioxide (NO$_{2)}$, ozone (O$_{3)}$, particulate matter (PM), and sulfur dioxide (SO$_{2)}$. EPA periodically  reviews the NAAQS to determine if revisions or supplements are warranted.  Title 40 CFR Part 50 – National Primary and Secondary Ambient Air Quality Standards,  http://www.ecfr.gov/cgi-bin/text-idx?c=ecfr&tpl=/ecfrbrowse/Title40/40cfr50_main_02.tpl.  5  Pollutant  Primary/  Secondary  Averaging  Time  Level  Form  Carbon Monoxide  (CO) 1  Primary  8-hour  9 ppm  Not to be exceeded more than once per year  1-hour  35 Lead (Pb) 2  Primary and  Secondary  Rolling 3  month  average  0.15 µg/m 3,7  Not to be exceeded  Nitrogen Dioxide  (NO$_{2}$) 3  Primary  1-hour  100 ppb  98$^{th }$percentile, averaged over three years  Primary and  Secondary  Annual  53 ppb 8  Annual mean  Ozone (O$_{3}$) 4  Primary and  Secondary  8-hour  0.075 ppm 9  Annual fourth-highest daily maximum 8-hr  concentration, averaged over three years  Particulate  Matter 5  PM$_{2.5 }$ Primary  Annual  12 µg/m 3  Annual mean, averaged over three years  Secondary  15 3  Primary and  secondary  24-hour  35 µg/m 3  98$^{th }$percentile, averaged over three years  PM$_{10 }$ Primary and  Secondary  24-hour  150 µg/m 3  Not to be exceeded more than once per year  on average over three years  Sulfur Dioxide  (SO$_{2}$) 6  Primary  1-hour  75 ppb 10  99$^{th }$percentile of 1-hour daily maximum  concentrations, averaged over three years  Secondary  3-hour  0.5 ppm  Not to be exceeded more than once per year  Table 2-1. National Ambient Air Quality Standards  Source: EPA,  National Ambient Air Quality Standards (NAAQS) , 2013, http://www.epa.gov/air/criteria.html.  Notes: ppb = parts per billion, ppm = parts per million, µg/m$^{3 }$= micrograms per cubic meter of air.  Federal Registers: $^{1 }$76 FR 54294, $^{2 }$73 FR 66964, $^{3 }$75 FR 6474 and 61 FR 52852, $^{4 }$73 FR 16436, $^{5 }$78 FR 3086, $^{6 }$75 FR  35520, and 38 FR 25678. $^{7 }$Final rule signed October 15, 2008. The 1978 lead standard (1.5 µg/m$^{3 }$as a quarterly average) remains in effect until one  year after an area is designated for the 2008 standard, except that in areas designated nonattainment for the 1978, the 1978  standard remains in effect until implementation plans to attain or maintain the 2008 standard are approved. $^{8}$The official level of the annual NO$_{2 }$standard is 0.053 ppm, equal to 53 ppb, which is shown here for the purpose of clearer  comparison to the 1-hour standard. $^{9 }$Final rule signed March 12, 2008. The 1997 ozone standard (0.08 ppm, annual fourth-highest daily maximum 8 -hour  concentration, averaged over 3 years) and related implementation rules remain in place.  In 1997, EPA revoked the 1-hour  ozone standard (0.12 ppm, not to be exceeded more than once per year) in all areas, although some areas have continued  obligations under that standard (“anti-backsliding”). The 1-hour ozone standard is attained when the expected number of days  per calendar year with maximum hourly average concentrations above 0.12 ppm is less than or equal to 1. $^{10 }$Final rule signed June 2, 2010 . The 1971 annual and 24 -hour SO$_{2 }$ standards were revoked in that same rulemaking .  6$_{ }$