There's another reason to eat fruits and veggies: A diet rich in them may lessen the harmful effects of air pollution for people suffering from chronic lung diseases.
Researchers looked at London hospital patients with asthma or chronic obstructive pulmonary disease (COPD), and found that those with low levels of vitamin C had an increased risk of breathing problems on days when outdoor air pollution levels were high.
"This study adds to a small but growing body of evidence that the effects of air pollution might be modified by antioxidants," said Michael Brauer, an environmental health scientist at the University of British Columbia in Canada.
Antioxidants, such as vitamin C, may protect the body from harmful molecules called free radicals that damage cells. Free radicals can form when air pollution enters the lungs, and evidence suggests they play a role in heart disease, cancer and even respiratory ailments.
Antioxidants can bind to free radicals, counteracting them before they damage cells.
The study is published in the July issue of the journal Epidemiology.
In the new study, researchers at Imperial College in London looked at more than 200 patients admitted to the hospital for asthma or COPD, along with the levels of air pollution on the days before and after they entered the hospital. The majority of patients were between ages 54 and 74, though some were as young as 18. Many of them were former smokers.
Specifically, the researchers looked at levels of "course particulate matter," which is produced largely through the combustion of fossil fuels.
Results showed that with every increase in course particulate matter of 10 micrograms per cubic meter (mcg/m3), there was a 35 percent increased risk of hospital admission for people with asthma or COPD.
However, the risk of admission was 1.2 times greater among people with low levels of vitamin C.
Previous studies have linked high levels of outdoor air pollution with an increased risk of deaths from respiratory illnesses, as well as emergency room visits for these conditions.
The U.S. Environmental Protection Agency sets the current air pollution standard as 50 mcg/m3 for course particulate matter, the same as the European standard. London air was above that standard on 45 days during the three-year study, though the median level during the study period was about 25 mcg/m3.
Particles made up of coarse particulate matter are about one-fifth the diameter of a human hair. Automobile exhaust and industrial sources, such as coal-burning power plants, are major contributors of particulate matter in the United States.
Why vitamin C may help
Citrus fruits and dark green vegetables such as broccoli and spinach are good sources of vitamin C.
It is certainly biologically plausible that vitamin C, which acts as an antioxidant, could protect against the effects of air pollution, said Patrick Ryan, an epidemiologist at the Cincinnati Children's Hospital Medical Center.
Study researcher Cristina Canova said, "the protective effect of vitamin C was still present after excluding smokers and elderly subjects, implying that the effect of this antioxidant was not explained by smoking or age." However, the study noted that smokers and older people tend to have lower levels of many nutrients than nonsmokers.
Because the study participants had either asthma or COPD, it's not clear whether vitamin C would benefit people without these ailments.
Most people get enough vitamin C in their daily diet, said Dr. Fernando Holguin, a pediatric pulmonologist at Children's Hospital of Pittsburgh, and taking high doses of the vitamin has never been shown to improve asthma.
If a person has an airway disease, he or she should continue to take their medications regularly and avoid outdoor activity on days with poor air quality, Holguin said.
Live Science newsletter
Stay up to date on the latest science news by signing up for our Essentials newsletter.