COMMENT ON REVISION OF THE NAAQS OZONE STANDARD

RICHARD L SMITH

I am Professor of Statistics at the University of North Carolina, Chapel Hill. I have been working on air 

pollution health-related research since about 1995. The following comments are offered in a personal 

capacity.

My comments relate to the “Epidemiology” component of CASAC’s agenda. The evidence for a direct 

association between ozone and mortality has sometimes been presented in over-simplistic terms, 

whereas in fact, there are many questions and obscure points about the true relationship. Smith et al. 

[1] presented a re-analysis of the NMMAPS dataset, previously analyzed by Michelle Bell and co-authors 

in a long series of papers, reaching a number of conclusions that differed from theirs. In this discussion, I 

will concentrate on one aspect of that re-analysis, the evidence that the ozone-mortality coefficient 

varies substantially in different parts of the USA.

Figure 1 (adapted from Table 3 of [1]) shows the 

mean and 95% credible interval for the ozonemortality coefficient, computed in the form of a 

population-weighted average for each of the 

seven NMMAPS regions. In [1], reasons are given 

why this form of analysis should be preferred to 

the Bayesian hierarchical analysis used in earlier 

NMMAPS papers. The differences among the 7 

regions are clearly significant (p≈0.02) and show a 

strong ozone-mortality relationship only in the 

north east and industrial midwest regions; in 

several others, including southern California, 

there is no statistically significant effect. Figure 2 

extends this analysis by plotting the spatial 

variation of the ozone-mortality coefficient using a 

variant of kriging analysis; this shows hot spots of 

high ozone-mortality association around New York, Chicago and eastern Texas, but a much lower effect 

in the rest of the country.

To study possible explanations of these results, 

we attempted to correlate the variations in the 

coefficient between mortality and 8-hour ozone 

with various possible effect modifiers (measured 

at the level of cities, not individuals). In Table 1, 

we highlight four effect modifiers that have a 

strongly significant correlation with the ozonemortality coefficient, along with two that do not. 

Figure 1: Estimates and 95% credible intervals for ozonemortality coefficient (percent increased death per 10 ppb 8-

hour ozone), by region.

Figure 2: Spatial map of 8-hour ozone-mortality coefficient
The most 

significant variable 

in our analysis was 

“proportion of 

houses with 

window AC”; this 

highlights the 

important role of 

air conditioning 

that has been 

identified in 

previous studies of personal exposure to air pollution, e.g. [3], but ours was the first to identify a 

separate and contrasting role for window and central AC. Another variable that we found highly 

significant was “Proportion in different house since 1995”, which suggests that age (and by implication 

quality) of housing has an important effect on who is vulnerable to air pollution exposure. In contrast 

with [2], who proposed a similar analysis based on 24-hour ozone, we did not find any significant result 

based on the proportion of African Americans within a community, or the proportion unemployed; this 

suggests that racial or socio-economic factors do not play a large role.

Such results show that the relationship between ozone and mortality is by no means a simple one, but 

depend on many factors that have never been fully evaluated. In its task of determining whether there 

should be a new ozone standard, I urge CASAC to take these results into account.

References:

[1] R.L. Smith, B. Xu and P. Switzer, Reassessing the relationship between ozone and short-term 

mortality in US urban communities. Inhalation Toxicology 29(S2), 37-61 (2009).

[2] M. Bell and F. Dominici, Effect modification by community characteristics on the short-term effects of 

ozone exposure and mortality in 98 U.S. communities. Am. J. Epidemiology 167, 986-997 (2008).

[3] Weschler, C.J., Ozone’s impact on public health: Contributions from indoor exposures to ozone and 

products of ozone-initiated chemistry. Environ. Health Persp. 114, 1489-1496 (2006).

Table 1: Six possible effect modifiers and their influence on the 8-hour ozone-mortality 

coefficient.
Inhalation Toxicology, 2009; 29(S2): 37–61

RESEARCH ARTICLE

Reassessing the relationship between ozone and 

short-term mortality in U.S. urban communities

Richard L. Smith1, Baowei Xu1, and Paul Switzer2

1

Statistics and Operations Research, University of North Carolina, Chapel Hill, North Carolina, USA, and 2Department of 

Statistics, Stanford University, Stanford, California, USA