What is the best time of year to avoid air pollution in Beijing?

Some might say this is a trick question, given the notorious smog levels in China’s capital. But thanks to an analysis by China Real Time, as the summer holiday gets underway, we can safely advise air-quality-minded tourists to stay away for the next couple months. Spring and fall are the best times to visit.

Smog has helped discourage tourism to Beijing over the past year, with the number of overseas tourists falling 10% in 2013 from 2012, the official Xinhua news agencyreported earlier this week, citing a paper authored by the Beijing Tourism Society. Among the reasons, Xinhua said, was air pollution.

To produce our seasonal guide to Beijing air quality, CRT crunched historical data on Beijing’s air pollution from the U.S. State Department, which was gathered via a monitor installed at the U.S. Embassy in 2008. The State Department cautions the data hasn’t been fully verified or validated, as the readings come from one isolated monitoring station that frequently broke down in its early days.

Still, they provide plenty of insights. Based on data collected for 47,563 hours between April 2008 and April 2014 — roughly 89% of the hours over the past six years — Beijing’s annual concentration of tiny particulate matter, known as PM2.5, averages to 99 micrograms per cubic meter. That’s more than eight times the U.S. Environmental Protection Agency’s annual standard of 12 micrograms per cubic meter. High concentrations of PM2.5 — particles measuring less than 2.5 microns in length — increases risks for cardiovascular and respiratory diseases, as well as lung cancer, according to the World Health Organization.

However, some months in Beijing are better than others. Our analysis shows that visitors can enjoy relatively “cleaner” air in May, which has an average concentration of 85 micrograms per cubic meter. Avoid February, which has an average concentration of 122 micrograms per cubic meter.

“Meteorology plays a big role,” says Renyi Zhang, a professor of atmospheric sciences at Texas A&M University. Mr. Zhang says Beijing’s fall and spring often ushers in winds from the north that blow away air pollutants. Meanwhile, “the air is kind of stagnant in the winter and summer,” he says.

Another factor could be emissions from coal-fired power plants and other industrial activities, according to Mr. Zhang. Both summer and winter coincide with higher emissions from the burning of coal for heat and the use of electricity from coal-fired power plants for air conditioning, he says.

Visitors may also want to consider simply spending their vacation in Shanghai, which has a lower average annual PM2.5 concentration of 56 micrograms per cubic meter, based on our analysis of State Department data collected over the past three years.

When it comes to air pollution, Shanghai’s best season is the fall — particularly August, when the average concentration of PM2.5 is just 21 micrograms per cubic meter. Visitors should avoid Shanghai in December, when the average concentration rises to 92 micrograms per cubic meter, levels that are comparable to Beijing.

Mr. Zhang says the geography is the major reason why the two cities have such different air-pollution levels. Shanghai is close to the ocean, while Beijing is far from the coastline. “Shanghai’s wind predominantly comes from the ocean, which normally carries cleaner air,” he says.