A “congestion tax” that discourages downtown driving not only cuts traffic and pollution, but also sharply reduces children’s asthma attacks.

The tax levied by Stockholm, Sweden, reduced air pollution levels between 5 and 10 percent, and eventually dropped the rate of pediatric asthma attacks by nearly 50 percent.

The health improvement appeared more gradually than the decline in pollution, suggesting that the full health benefits from pollution cuts don’t occur immediately, says economist Emilia Simeonova, assistant professor at Johns Hopkins University’s Carey Business School.

The drop in the rate of asthma attacks was more than 12 percent during the first seven months of the tax but soared to 47 percent after a few years of implementation.

“The key takeaways of this paper are that health gains can be realized through efforts to lower air pollution, and that we need to be patient in waiting for the complete picture to emerge.”

Asthma, the chronic inflammation of breathing passages, afflicts people of all ages. Its onset in childhood, however, can lead to poor lung development, causing ill effects―wheezing, breathlessness, chest tightness, and coughing―that can recur over a lifetime. Asthma is the leading cause of hospitalization among children in the United States, especially those living in densely populated areas with frequent traffic congestion.

For the study, researchers examined official health and environmental data compiled in Sweden from 2004 through 2010. They focused on health statistics for children up to age six, who tend to experience the most acute asthma episodes because their families haven’t yet learned how to tame the flare-ups.

Stockholm’s congestion tax began as an experiment, from January through July 2006. Traffic was reduced 20 to 25 percent. Deeming the trial a success, the city government reinstituted the congestion tax in August 2007 and it has been in place ever since.

The tax costs drivers up to $2.60 (in the equivalent of US dollars) per vehicle, depending on the time of day. There are no charges at night, on weekends and public holidays, or during July. Tolls are automatically assessed via scanners that collect license plate information from cars crossing into the “congestion pricing zone.”

A positive impact on health was evident from the start. During the seven-month trial period, visits to doctors for asthma symptoms fell from a baseline of 18.7 per 10,000 children to 16.4, a drop of 12 percent. For about a year after the trial period, the congestion tax wasn’t in effect. Pollution levels rose again slightly, though not up to pre-trial levels―and asthma visits among children continued to fall, to 13.9 per 10,000, or 26 percent below baseline.

A few years after the tax was made permanent, visits had dropped even further, to 10 per 10,000 children, a reduction of 47 percent from the baseline. (The baseline figure of 18.7 asthma visits was an average of statistics from a two-year period before the trial.)

During the “in between” period when the tax wasn’t being levied, the level of pollution still wasn’t high enough to reverse the health benefits that Stockholm’s children had begun to realize, Simeonova says. But, the rate of asthma visits probably would have started climbing if the tax had not been made permanent.

“These findings show that traffic congestion fees in large cities can have significantly positive effects on health in the short-term, but even larger effects in the longer term,” Simeonova says.

Stockholm’s average pollution levels are not nearly as bad as levels judged acceptable by the US Environmental Protection Agency, the economist says. Pollution reductions, then, even in a city of relatively mild air quality problems can produce benefits for respiratory health, especially among young children.

The authors presented the study at January’s annual meeting of the American Economic Association. Coauthors are from UC Berkeley, Stockholm University, and Princeton University. The Swedish Research Council provided funding.