专四专八考试

【单选题】

Seven years ago, anEnvironmental ProtectionAgency 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 inEPA’s decision last year to ratchet down the permitted concentration of breathable pm-tides in urban air -- and in human airways.
At the time, many industrialism 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 ScienceCenter 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.
Andfij 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 MarchEnvironmental 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 suppressor 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 upstream event and prevent that injury."
"This is, I think, an important contribution to the overall story," saysDaniel L.Costa ofEPA’s pulmonary toxicology branch in Research Triangle Park, N.C、
Studies byEPA、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 "preeminent" role in the toxicity of airborne particulates. WhenEPA、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 ofEPA’s National Health andEnvironmentalEffects Research Laboratory in Research Triangle Park, says Holian’s results are "a nice complement to our studies."

Which of the following statements about
网考网参考答案：A
网考网解析：
该题问：下列哪一项关于人类肺泡中的巨噬细胞的陈述是正确的 见本文第七段第三句...moat of the suppressor macropbages underwent apoptosis，or cellular suicide，while the stimulatory ones survived unaffected．可知选项A正确。B项意思正好相反。从第六段可知，肺部健康的人的suppressor macrophages与stimulatory macrophages的比例是10比1，有呼吸疾病的人则是3比1，C项意思相反。 document.getElementById("warp").style.display="none"; document.getElementById("content").style.display="block"; 查看试题解析出处>>