Smog in Los Angeles
Congested freeways crawling with cars and trucks are notorious for causing smog in Los Angeles， but a new study finds that heavy airplane traffic can contribute even more （1） p ， and the effect continues for up to 10 miles away from the airport. This may seriously affect the health of residents near Los Angeles International Airport （LAX） and other airports around the world.
American scientists note that past research has （2） m pollution from air traffic before， but most of these studies only sampled air within a couple of miles， at most， from airports. Not surprisingly， these （3） a have found higher levels of pollutants， such as nitrogen oxides and small （ultrafine） particles less than 0.1 micron （about one-thousandth of the width of a human hair）， that scientists attributed to airplane emissions.
This added pollution is potentially a major public （A） h issue. Ultrafine particles are of particular concern because they deposit deeply into the lungs and can enter the bloodstream. They appear to play a role in the development of blocked arteries and can make other health conditions worse， （B） e for people with existing cardiac（心脏病的）or lung conditions including asthma（气喘）.
Scientists suspect that （C） r near LAX， the sixth busiest airport in the world， were getting exposed to excessive doses of pollution from airplanes even farther from the runways than previous research had considered. During its busiest times， 40 to 60 jets take off and land every hour.
Over a period of 29 days， the scientists drove the area within 10 miles downwind of the airport to measure levels of air pollutants. They found that over a 23-square-mile area， particle number （PN） concentrations were double the background levels. Over 9 （D） s miles， levels were five times higher than background. And within nearly 2 miles east of the airport， PN levels were nearly 10 times higher.
Based on their calculations， scientists concluded that within the area they found to have elevated pollution from the airport， automobiles contributed less than 5 （4） p of the PN levels. Therefore， the LAX should be considered one of the most important sources of PN in Los Angeles， the scientists suggest.（A， B， C， D FOR CROSS， 1， 2， 3， 4 FOR DOWN. The first letters of the absents were given）