News and Analysis Scientific American April 1997 27

are rapidly finding better methods forprotecting neurons against excitotoxic-ity, a process in which overactive pro-teins poison cells. And they are devel-oping tactics to halt programmed celldeath, or apoptosis. “The two pathwaysmay occur in parallel in ischemia,” Choisays, “and so we may need to developcombined drug interventions.”

In hopes of stalling excitotoxicity, sci-

entists have long tracked the effects ofglutamate. This neurotransmitter floodsthe brain within hours after injury andopens NMDA receptors, porelike mole-cules that help to regulate the flow ofcharged ions in and out of brain cells.When NMDA receptors are overstimu-lated, they stay open, and affected neu-rons swell with toxic levels of sodiumand calcium. Many cells die, but the

natural acidity in the brain after astroke typically turns NMDA receptorsoff within minutes—which presumablyhelps to keep the total damage in check.

Recently researchers have solved thispuzzle with the discovery that gluta-mate-induced cell death can also be me-diated primarily by other receptors,called AMPAs. Save during brief mo-ments in fetal development, AMPA re-

The worst air pollution disaster ever recorded was in De-cember 1952, when a temperature inversion trapped

soot, sulfur dioxide and other noxious gases over London,killing 4,000. Nothing as dramatic has ever happened in a U.S.city, nor is it likely to, thanks largely to the efforts of the Envi-ronmental Protection Agency and various state agencies. Still,it is likely that thousands of Americans die prematurely everyyear because of air pollution.

The EPA has focused on air concentrations of six pollutants:ozone, nitrogen dioxide, sulfur dioxide, carbon monoxide,particulates (soot) and lead. (The concern here is ground-lev-el ozone, not ozone in the stratosphere, which blocks ultravioletrays.) The first five adversely affect lung function, exacerbat-ing problems such as asthma. In addition, carbon monoxide,sulfur dioxide and particulates contribute to cardiovasculardisease; the last also promotes lung cancer. Lead causes men-tal retardation in children and high blood pressure in adults.Nitrogen oxides and sulfur dioxide are the principal contribu-tors to acid rain, and ozone damages crops and trees.

For each pollutant, the EPA has designated a maximum airconcentration compatible with good health. The map showsareas where concentrations of the six pollutants were abovethe maximum in September 1996, a fairly typical period.Southern California has long had the biggest problems, withLos Angeles, for example, having 103 days during 1995 in

which one or more pollutants exceeded the standard. Still,this level marks an improvement over the 239 days recordedin 1988. In contrast, no metropolitan area east of the Missis-sippi registered more than 19 days above the maximum, andalmost half registered two days or fewer. Over the past de-cade or so, air quality in the East has improved, but ozone andseveral other pollutants remain substantial problems in manyareas. Stringent new standards for ozone and particulatesproposed by the EPA for adoption later this year would resultin many new areas failing to comply. These areas are mostlyeast of the Mississippi, with the East North Central and MiddleAtlantic states, the Carolinas, Tennessee and Kentucky beingparticularly affected.

The graph shows the dramatic fall in lead emissions since1970, which stems from the elimination of leaded gasoline.Emissions of the other pollutants, with the exception of nitro-gen oxides, have been on a downward trend since the early1970s. Air concentrations of the six pollutants are also head-ing down, except for ozone, which is rising. Ozone, now themost widespread air pollutant, is not emitted directly butemerges from the interaction of other gases, notably nitrogendioxide and volatile organic compounds. In 1995, 47 percentof emissions of the six pollutants came from transportation,mostly motor vehicles; another 26 percent was of industrialorigin. —Rodger Doyle

1900

EMIS

SIO

NS

OF

POLL

UTA

NTS

(MIL

LIO

NS

OF

SHO

RT T

ON

S)

100

NITROGENOXIDES

SULFUR DIOXIDE

OZONE

CARBON MONOXIDE

1920 1940 1960YEAR

1980 2000

10

1

NITROGEN DIOXIDE

LEAD

PARTICULATES

VOLATILE ORGANICCOMPOUNDS

SOURCE: Environmental Protection Agency. Map shows where air concentrations of sulfur dioxide, particulates, carbon monoxide, lead, ozone and nitrogen dioxide exceeded EPA standards during September 1996. The graph shows the emissions of the first four plus nitrogen oxides and volatile organic compounds.

B Y T H E N U M B E R S

Air Pollution in the U.S.

ROD

GER

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Copyright 1997 Scientific American, Inc.