NEJM: PM-10 pollution & mortality, 20 US Cities

Gary Greenberg Gary.Greenberg at Duke.edu
Wed Dec 13 22:37:50 EST 2000

The New England Journal of Medicine 
December 14, 2000 
Vol. 343, No. 24


Fine Particulate Air Pollution and Mortality in 20 U.S. Cities,

Jonathan M. Samet, Francesca Dominici, Frank C. Curriero, Ivan Coursac,
Scott L. Zeger 


Air pollution in cities has been linked to increased rates of mortality
and morbidity in developed and developing countries. Although these
findings have helped lead to a tightening of air-quality standards,
their validity with respect to public health has been questioned. 

We assessed the effects of five major outdoor-air pollutants on daily
mortality rates in 20 of the largest cities and metropolitan areas in
the United States from 1987 to 1994. The pollutants were particulate
matter that is less than 10 µm in aerodynamic diameter (PM10), ozone,
carbon monoxide, sulfur dioxide, and nitrogen dioxide. We used a
two-stage analytic approach that pooled data from multiple locations. 

After taking into account potential confounding by other pollutants, we
found consistent evidence that the level of PM10 is associated with the
rate of death from all causes and from cardiovascular and respiratory
illnesses. The estimated increase in the relative rate of death from all
causes was 0.51 percent (95 percent posterior interval, 0.07 to 0.93
percent) for each increase in the PM10 level of 10 µg per cubic meter.
The estimated increase in the relative rate of death from cardiovascular
and respiratory causes was 0.68 percent (95 percent posterior interval,
0.20 to 1.16 percent) for each increase in the PM10 level of 10 µg per
cubic meter. There was weaker evidence that increases in ozone levels
increased the relative rates of death during the summer, when ozone
levels are highest, but not during the winter. Levels of the other
pollutants were not significantly related to the mortality rate. 

There is consistent evidence that the levels of fine particulate matter
in the air are associated with the risk of death from all causes and
from cardiovascular and respiratory illnesses. These findings strengthen
the rationale for controlling the levels of respirable particles in
outdoor air. (N Engl J Med 2000;343:1742-9.) 

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The New England Journal of Medicine -- December 14, 2000 -- Vol. 343,
No. 24 



Particulate Air Pollution and Mortality -- Clearing the Air

In the years after World War II, several episodes of severe air
pollution in the United States and Britain aroused public concern about
the effects on health of air pollutants produced by burning fossil
fuels. ...

In the United States, concentrations of particulate air pollution have
declined since the early 1970s. From 1988 to 1993, the average of the
annual mean PM10 concentrations at 799 sites monitored by the EPA
declined by 20 percent. (1) Despite these improvements in air quality, a
series of studies (2,3,4) reported associations between particle
concentrations and the numbers of deaths per day in several U.S. cities
with mean 24-hour PM10 concentrations well below the standard.
Responding to a substantial body of epidemiologic evidence, the EPA
wrote in 1996 that the "staff can not conclude that the current
standards protect public health with an adequate margin of safety" and
that "fine fraction particles [PM2.5, or particles with a diameter of
less than 2.5 µm] are a better surrogate for those particle components
linked to mortality and morbidity effects at levels below the current
standards." (1) In 1997, the EPA retained the PM10 standards and
promulgated new 24-hour and annual standards for PM2.5, of 65 and 15 µg
per cubic meter, respectively, based on consistency with the literature
on health effects. 

Both the epidemiologic evidence and the new PM2.5 standard have been
criticized. Some observers have asserted that the associations found in
the epidemiologic studies are weak, inconsistent, and attributable to
confounding by weather, other pollutants, or misclassification of the
exposure to particulate matter. (5) They note that many of the studies
were performed by the same small group of investigators, that the study
cities were not selected systematically, and that the statistical models
varied from study to study. 

Arguing that the 1997 standards for ozone and particulate matter did not
have an adequate scientific basis, industry groups sued the EPA in the
Court of Appeals for the District of Columbia. In 1999, the court
blocked implementation of the 1997 standards. In its current session,
the Supreme Court is hearing an appeal of this decision by the EPA. 

A key issue before the Court is whether the cost of compliance can be
considered in setting the standard. The EPA has estimated that
compliance with the 1997 standards for PM2.5 and ozone will require an
investment of about $10 billion per year to modify power plants, diesel
trucks, and other sources of these pollutants (6) and will result in
health benefits with an estimated value of $20 billion to $100 billion
per year. Others have estimated that the costs of compliance could be as
high as $60 billion per year. (7) Thus, the Supreme Court's decision
could have substantial consequences for the economy and the public's
health. Given these stakes, the public and the scientific community need
more and better information about the health effects of particulate air

The study reported by Samet et al. (8) in this issue of the Journal
(along with the more extensive investigation from which it is derived
(9)) strengthens our understanding of the epidemiologic evidence and
addresses the criticism of earlier work. The investigators used a single
analytic approach to examine the association between PM10 concentrations
in a given 24-hour period and the numbers of deaths reported on the
following day in 20 of the largest cities and metropolitan areas in the
United States. Samet et al. found an average increase in the rate of
death from all causes of about 0.5 percent for every increase in the
PM10 concentration of 10 µg per cubic meter. 

The PM10 concentrations were positively associated with daily mortality
rates in most of the 20 cities studied and at concentrations well below
the current 24-hour standard of 150 µg per cubic meter. In fact, the
90th percentile of the distribution of daily values was below the
24-hour standard in each of the 20 cities. Moreover, the association was
specific to PM10. The concentrations of other regulated air pollutants
produced by the combustion of fossil fuels (sulfur dioxide, nitrogen
dioxide, and carbon monoxide) were weakly and inconsistently associated
with daily mortality rates. Though ozone concentrations were positively
associated with daily mortality rates during the summer months, this
relation did not influence the association between the PM10
concentration and the daily mortality rate. Finally, the finding of a
strong association between the PM10 concentration and the rate of death
from cardiovascular and respiratory causes offers support for the idea
that the concentrations of particulate air pollution influence

The findings of Samet et al. are consistent with those of time-series
studies in Europe (10) and cohort studies in the United States. (11)
Thus, the evidence in support of an association between the
concentration of particulate air pollution and the mortality rate is
consistent, is not affected by differences in statistical methods, and
can be generalized. 

There are important gaps in both the scientific evidence of causation
and the scientific basis for the regulatory response. The most important
is our inability to explain how fine particles affect health. Some
studies have found that the daily mortality rate is associated with the
concentration of fine particles (PM2.5) but not coarse mass (PM10 -
PM2.5). (12) These findings are consistent with the evidence that fine
particles penetrate indoor spaces, are chemically active, and are
deposited in the respiratory bronchioles and alveoli. Yet little is
known about the specific constituents or characteristics of PM2.5 that
adversely affect health. Moreover, although the standard proposed in
1997 is based on concentrations of PM2.5, most of the epidemiologic
evidence has been obtained from measurements of PM10 or other, less
relevant indicators. Lacking knowledge of the harmful constituents of
fine particles and the mechanisms by which they affect health, the EPA
continues to propose standards based on particle mass. 

The epidemiologic evidence suggests that the association between
fine-particle concentrations and mortality is linear across the entire
range of current concentrations. Although substantial reductions can be
achieved at a reasonable cost, a reduction in 24-hour exposures to
levels consistently below the current range would be prohibitively
costly, if not impossible, in the foreseeable future. An aggressive
research program to identify the harmful components of PM2.5, their
sources, and the mechanisms of their effects offers the best hope for
developing more focused regulatory strategies that will simultaneously
protect the public health and the nation's prosperity. In the meantime,
these results present a challenge to policy makers who are required to
protect the public's health with an adequate margin of safety. 

James H. Ware, Ph.D. Harvard School of Public Health Boston, MA 02115 

Gary N. Greenberg, MD MPH    Sysop / Moderator Occ-Env-Med-L MailList
gary.greenberg at duke.edu     Duke Occupat, Environ, Int & Fam Medicine
OEM-L Maillist Website:                      http://occhealthnews.com


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