Cold viruses and certain pollutants, researchers say, both prompt cells to release the kinds of inflammatory agents that make people feel lousy. When the two are combined, the total is greater than the sum of its parts.
The effects "may add together and pile on top of one another," said William Spannhake, associate chairman of the Department of Environmental Health Sciences at the Johns Hopkins Bloomberg School of Public Health in Maryland.
The findings are reported in the July issue of the journal Environmental Health Perspectives.
Dr. James E. Gern of the University of Wisconsin-Madison called the research new and potentially useful.
A relationship between the effects of pollution and some viruses has been noted in the past, but not between pollution and the common cold, said Gern, who was not part of the research team.
"We've all been looking for things that seem to make common colds worse, because some people seem to have a lot of complications, sinus disease and asthma, and other people don't," Gern said. "It's extremely productive to look at this sort of research into second factors that seem to make colds worse."
When human cells confront pollutants or cold viruses they release cytokines, chemicals that cause inflammation, leading to release of fluids, swelling and other symptoms.
Spannhake was studying asthmatics, who already have a low level of inflammation in their airways.
"We were looking at two environmental stimuli, one chemical and one biological, that have the capacity to enhance that existing inflammation," he explained.
"We found that these two stimuli, which on the surface appear to be quite different, lead to the same type of increase in" inflammation.
This raises obvious concerns for people with asthma, but also for others who may have colds and face days with high pollution, he said.
To determine the effect, the researchers used nasal tissues taken from patients undergoing surgery.
They isolated cells from these normal tissues and infected some of them with cold virus. Infected and uninfected cells were exposed to normal air or air containing various concentrations of nitrogen dioxide or ozone.
When the cells were faced with both the virus and one low dose of nitrogen dioxide — one part nitrogen dioxide per 3,000 parts air, they released 41 percent more cytokines than one would expect from simply adding the effects of virus and nitrogen dioxide administered separately, the researchers found.
When the nitrogen dioxide was more concentrated — one part per 1,000 parts air — the cytokine release that was 191 percent more than the additive effect of the two agents administered alone.
At one part in 300, the boost in cytokines was about 250 percent.
When virus and ozone were combined the increases in cytokine release were not as great, but still ranged between 41 percent and 67 percent above that expected from the responses to virus and ozone administered separately.
The reason for such high levels of cytokine release is not yet clear, Spannhake said.