NO2 Primary NAAQS –

Overview of Final Rulemaking

1

Briefing for OMB

December 16, 2009

Office of Air and Radiation

Office of Air Quality Planning and Standards
Background

• Current NO2 NAAQS: Annual standard, level is 53 ppb, all areas of U.S. are in

attainment

• Existing NO2 monitors (about 400 total) are primarily sited to measure area-wide

concentrations 

• NO concentrations can be 30-100% higher near roads than away from the road

2

• NO2concentrations can be 30-100% higher near roads than away from the road

• Individuals who spend time on or near major roadways (residence, schools,

commuting) can experience short-term NO2exposures higher than measured 

by current network

• Schedule for ongoing review 

– December 16, 2009: Send draft of rule to OMB for interagency review 

– January 22, 2010: Court schedule requires signature of final rule 
Key Conclusions from the ISA and REA

• The evidence relating short-term (minutes to hours) NO2exposures to respiratory morbidity was judged to be 

sufficient to infer a likely causal 

relationship

• This conclusion is supported by epidemiologic and experimental evidence 

• Epidemiologic studies (1- to 24-hours)

– Generally report positive associations with respiratory morbidity endpoints, with a number statistically significant

3

– Generally report positive associations with respiratory morbidity endpoints, with a number statistically significant

• Many of these studies were conducted in areas with NO2concentrations below those allowed by the current NAAQS 

– Evidence for respiratory effects is consistent (associations reported in numerous locations with variety of 

methodological approaches) and coherent (respiratory health outcomes are logically linked together) 

– NO2effect estimates generally remain robust in multi-pollutant models 

• Experimental studies (30-minutes to 3-hours)

– Human clinical and animal toxicological studies support the plausibility of the link between NO2and respiratory morbidity 

reported in epidemiologic studies 

– Provide evidence for effects of NO2on lung host defense and immunity, airway inflammation, airway response 

– A meta-analysis of human clinical studies reported that 

small but significant increases in nonspecific airway hyperresponsiveness were

observed

at 0.1 ppb NO2for 60-min exposures in

asthmatics.
Overview of Final NO2 NAAQS 

• Administrator judges that current annual standard alone is not requisite to

protect public health with an adequate margin of safety, based 

on

– Epidemiologic evidence for NO2-associated respiratory effects in locations where 

annual average NO2concentrations were below the level of the current annual NO2

4

annual average NO2concentrations were below the level of the current annual NO2

NAAQS 

– Supported by REA analyses: Policy assessment chapter concluded that risks 

estimated to be associated with air quality adjusted upward to simulate just meeting 

the current standard can reasonably be concluded to be important from a public 

health perspective 

– CASAC

concurs with

EPA

s judgment that the current NAAQS does not protect the 

public
Overview of Final NO2 NAAQS (Continued)

• Level of new 1-hour standard: 10 pb

– Based on both human clinical and epidemiologic studies, considered within the context of available 

information on the NO2concentration gradient around roads

• Clinical studies:The Administrator concludes that the NO2-induced increases in airway 

responsiveness reported for exposures to NO2concentrations at or above 100 ppb could be

adverse for some asthmatics; however, she also notes that important uncertainties exist with 

regard to the extent to which the reported effects are adverse 

– Therefore, she concludes that a standard level no higher than 100 ppb is appropriate to reflect a 

5

– Therefore, she concludes that a standard level no higher than 100 ppb is appropriate to reflect a 

cautious approach to considering the uncertainty associated with the NO2-induced increase in airway 

responsiveness, but that this uncertainty also supports setting a standard level no lower than 100 ppb 

• Epidemiologic studies:Based on available information on the NO2concentration gradient 

around roads, a standard level of 100 ppb for a standard reflecting the maximum allowable 

NO2concentration anywhere in the area would be expected to maintain area-wide NO2

concentrations below those measured in locations where key U.S. epidemiologic studies have 

reported associations with respiratory-related emergency department visits and hospital 

admissions 

– A group of 5 key U.S. epidemiologic studies provide evidence for associations between NO2and 

respiratory-related emergency department visits and hospital admissions in locations where 98th 

percentile 1-hour daily maximum NO2concentrations measured at area-wide monitors ranged from 85 

to 94 ppb 
Overview of Final NO2 NAAQS (Continued)

• Form: 98

th percentile of the 3-year average of the annual distribution of 1-hour daily 

maximum NO2concentrations 

• Monitoring network:To measure maximum NO2concentrations, a 2-tiered NO2

monitoring network will be established, including both near-road and area-wide 

monitors

– Near-road monitors will be required within 50 meters of the roads expected to cause the 

maximum NO concentrations in an area (considering traffic volume, fleet mix, road 

6

maximum NO2concentrations in an area (considering traffic volume, fleet mix, road 

design, etc.) 

– 167 near-road monitors in 143 areas and 52 area-wide monitors

• 1 near-road monitor in Core Based Statistical Areas (CBSAs) with 350,000 or more people

• A second near-road monitor in CBSAs with 2.5 million people or more OR any CBSA that has 

one or more road segments with 250,000 Annual Average Daily Traffic (AADT) counts

• 1 area-wide monitor in CBSAs with 1 million or more people 

– Additional monitors may be required, at the discretion of the Regional Administrator, for 

example, if maximum concentrations are expected to occur in locations other than near 

a major road (e.g., multiple smaller roads and/or stationary sources)