1
News and Analysis Scientific American April 1997 27 are rapidly finding better methods for protecting neurons against excitotoxic- ity, a process in which overactive pro- teins poison cells. And they are devel- oping tactics to halt programmed cell death, or apoptosis. “The two pathways may occur in parallel in ischemia,” Choi says, “and so we may need to develop combined drug interventions.” In hopes of stalling excitotoxicity, sci- entists have long tracked the effects of glutamate. This neurotransmitter floods the brain within hours after injury and opens NMDA receptors, porelike mole- cules that help to regulate the flow of charged 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 sodium and calcium. Many cells die, but the natural acidity in the brain after a stroke typically turns NMDA receptors off within minuteswhich presumably helps to keep the total damage in check. Recently researchers have solved this puzzle 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- T he 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 every year 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 ultraviolet rays.) The first five adversely affect lung function, exacerbat- ing problems such as asthma. In addition, carbon monoxide, sulfur dioxide and particulates contribute to cardiovascular disease; 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 air concentration compatible with good health. The map shows areas where concentrations of the six pollutants were above the maximum in September 1996, a fairly typical period. Southern California has long had the biggest problems, with Los 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 recorded in 1988. In contrast, no metropolitan area east of the Missis- sippi registered more than 19 days above the maximum, and almost half registered two days or fewer. Over the past de- cade or so, air quality in the East has improved, but ozone and several other pollutants remain substantial problems in many areas. Stringent new standards for ozone and particulates proposed by the EPA for adoption later this year would result in many new areas failing to comply. These areas are mostly east of the Mississippi, with the East North Central and Middle Atlantic states, the Carolinas, Tennessee and Kentucky being particularly affected. The graph shows the dramatic fall in lead emissions since 1970, 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 early 1970s. Air concentrations of the six pollutants are also head- ing down, except for ozone, which is rising. Ozone, now the most widespread air pollutant, is not emitted directly but emerges from the interaction of other gases, notably nitrogen dioxide and volatile organic compounds. In 1995, 47 percent of emissions of the six pollutants came from transportation, mostly motor vehicles; another 26 percent was of industrial origin. Rodger Doyle 1900 EMISSIONS OF POLLUTANTS (MILLIONS OF SHORT TONS) 100 NITROGEN OXIDES SULFUR DIOXIDE OZONE CARBON MONOXIDE 1920 1940 1960 YEAR 1980 2000 10 1 NITROGEN DIOXIDE LEAD PARTICULATES VOLATILE ORGANIC COMPOUNDS 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. BY THE NUMBERS Air Pollution in the U.S. RODGER DOYLE Copyright 1997 Scientific American, Inc.

Air Pollution in the U.S

  • Upload
    rodger

  • View
    213

  • Download
    1

Embed Size (px)

Citation preview

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

DO

YLE

Copyright 1997 Scientific American, Inc.