Associations between NO₂ levels and asthma-related emergency room visits

The pilot project analysis indicates that, among children, the risk of emergency room (ER) visits due to asthma was associated with increased levels of NO₂, which is a compound found in traffic exhaust.

  For all of the children in the sample, we found that living in areas with extremely high NO₂ levels (top 5% of children in the sample) is associated with a 40% increase in risk for visiting the ER for asthma.

We know from our asthma findings that many of the children who visited the ER for asthma lived in low-income neighborhoods.  Therefore, the increase in risk associated with increased NO₂ levels may actually be due to other factors associated with living in a low-income neighborhood (besides pollution exposure). 

Were there associations among children taking medication?

Children who receive maintenance medications are likely to have moderate-to-severe asthma and are already being seen by a clinician (doctor, nurse, etc.).   Among children who receive maintenance medications:

  Living in areas with high NO₂ levels (top 25% of children receiving maintenance medications) is associated with a 40% increase in risk for visiting the ER for asthma.

  Living in areas with extremely high NO₂ levels (top 1% of children receiving maintenance medications) may be associated with more than a 300% increase in risk for visiting the ER for asthma.

Only some of this risk is likely to be due to other factors associated with living in a low-income neighborhood, since some of the effect seems to occur in both high- and low-income neighborhoods.

How much NO₂ is that?

The National Ambient Air Quality Standard for NO₂ (annual average) is set by the EPA.  This limit was set to protect public health, including "sensitive populations" such as asthmatics, children, and the elderly.  The standard states that the annual average level of NO₂ cannot exceed 53 ppb (parts per billion). 

In 2003, the Alameda County maximum annual average was 17 ppb.  Based on our estimates of  NO₂, we found that for Alameda County in 2001:

  Increased risk for asthma ER visits was associated with annual  NO₂ levels estimated to be 20 ppb and greater.

  25% of the children in our sample were exposed to annual  NO₂ levels estimated to be above this level (20 ppb).

The actual level of  NO₂ may be higher or lower than our estimates.  We predict levels may be 2.8 ppb higher or lower than what we estimated using our model.  However, the important point to remember is that the differences between asthma outcomes at higher and lower NO₂ levels are real.

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Associations between birth outcomes and traffic pollution

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