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  3. Seven years ago, an Environmental Protection Agency statistician stunned researchers studying the effects of air pollution on he

Seven years ago, an Environmental Protection Agency statistician stunned researchers studying the effects of air pollution on he

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问题    Seven years ago, an Environmental Protection Agency statistician stunned researchers studying the effects of air pollution on health when he reported analyses indicating that as many as 60, 000 U. S. residents die each year from breathing federally allowed concentrations of airborne dust. This and subsequent studies figured prominently in EPA’s decision last year to ratchet down the permitted concentration of breathable particles in urban air -- and in human airways.
   At the time, many industrialists argued that they shouldn’t have to pay for better pollution control because science had yet to suggest a plausible biological mechanism by which breathing low concentrations of urban dust might sicken or kill people.
   Now, scientists at the University of Texas Houston Health Science Center describe how they uncovered what they think may be one of the basic elements of that toxicity.
   On the alert for foreign debris, a community of white blood cells known as alveolar macrophages patrols small airways of the lung. When these cells encounter suspicious material, they identify it and send out a chemical clarion call to rally the immune system cells best suited to disabling and disposing of such matter.
   The trick is to recruit only as many troops as are needed. If they call in too many, the lung can sustain inflammatory damage from friendly fire. Alongside the small troop of macrophages that stimulates defense measures, a larger squadron of macrophages halts immune activity when it threatens the host.
   Andrij Holian and his coworkers in Houston have found that people with healthy lungs normally have 10 times as many suppressor macrophages as stimulatory ones. In people with asthma and other chronic lung diseases -- who face an increased risk of respiratory disease from inhaling urban dust -- that ratio may be only 3 to 1. The reason for the difference is not known.
    In a report to be published in the March Environmental Health Perspectives, Holian’s team describes test-tube studies of human alveolar macrophages. The macrophages showed no response to ask collected from the Mount St. Helen’s eruption. However, when exposed to airborne dust from St. Louis and Washington, D. C. , most of the suppresser macrophages underwent apoptosis, or cellular suicide, while the stimulatory ones survived unaffected. Ash from burned residual oil, a viscous boiler fuel, proved even more potent at triggering suppressor cell suicides.
   It this test-tube system models what’s actually happening in the human lung, Holian told Science News, the different responses of the two classes of lung macrophages could result in an overly aggressive immune response to normal triggering events. Indeed, he says, it would be the first step in a cascade that can end in inflammatory lung injury. "We may one day be able to target this up stream event and prevent that injury."
   "This is, I think, an important contribution to the overall story," says Daniel L. Costa of EPA’s pulmonary toxicology branch in Research Triangle Park, N.C.
   Studies by EPA suggest that certain metals -- especially iron, vanadium, nickel, and copper -- in smoke from combustion of fossil fuels trigger particularly aggressive inflammatory responses by lung cells. Costa says these metals play a "preminent" role in the toxicity of airborne particulates. When EPA researchers removed the metals, they also removed the toxicity, he says. Moreover, he notes, these metals tend to reside on the smallest water-soluble particles in urban air --the fraction targeted for more aggressive controls under the new rules.
   John Vandenberg, assistant director of EPA’s National Health and Environmental Effects Research Laboratory in Research Triangle Park, says Holian’s results are "a nice complement to our studies."
According to this passage, Environmental Protection Agency ______.
选项 A、is the only agency studying the effects of air pollution on health
B、has launched a surprised attack on researchers
C、has decided to decrease the permitted concentration of airborne dust
D、has tremendously improved pollution control
答案C
解析 该题问：据本文介绍，环保局采取了何种举措？由第一段第四行... in EPA’s decision last year to ratchet down the permitted concentration of breathable particles in urban air... (去年，环保局决定降低城市空气中所含尘埃粒子的许可含量)可知，选项C的has decided to decrease the permitted concentration of airborne dust正确，应为正确答案。选项A意为“环保局是研究空气污染对健康影响的惟一机构”。本文中没有提到此处所说的“only’’(惟一)的意思，所以，选项A不正确；本文第一句只提到... an Environmental Protection Agency statistician stunned researchers... (环保局一名统计员的统计使研究人员十分震惊)，本文并没有说“环保局出其不意地攻击研究人员”，所以，选项B不正确；本文没有提到“环保局极大提高了控制污染的水平”，所以，选项D不正确。
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