In 2009, Italian authorities installed a heating, ventilation (通风) and airconditioning system to protect the painting from Milan's polluted air. To see whether the system could improve air quality, a team of researchers led by Constantinos Sioutas, a professor of civil and environmental engineering at the University of Southern California, placed two airquality monitors inside the church for a year to collect samples from around painting. Results showed the levels of fine and coarse particulates were reduced near the painting by 88 percent and 94 percent, respectively, compared with the corresponding outdoor levels. “It's a spectacular reduction，” Sioutas said in a statement. “It is, frankly, very impressive.”

　　Although the researchers applauded the successful decrease in particulates around da Vinci's painting, they warned that indoor sources of pollution may still pose a threat to the priceless painting. Fatty lipids (油脂) from the skin of customers in the church appeared in significant quantities around the painting, despite the fact that visitors getting access to the artwork are strictly regulated.

　　Only a few visitors are allowed inside the church at a time, and they enter the building and can stay for only 15 minutes each. However, fatty lipids from visitors' skin can combine with dust in the air and pollute the painting if they come in contact with it, says study researcher Nancy Daher of the University of Southern California.

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