The main pollen culprits tied to the increase in hospital visits came from oaks and grasses, whose levels spike in the spring.
"For people who have an allergy component to their asthma, using the tools of pollen surveillance and public health warning systems...can help with behavioral management of symptoms for people who are affected by these diseases," said Lyndsey Darrow, the lead author of the study and an assistant professor at Emory University's Rollins School of Public Health.
This might include staying indoors on days when pollen concentrations are elevated.
Darrow said it's been known that pollen can exacerbate individuals' asthma symptoms, but little was known about what this means on a population-wide scale.
She and her colleagues analyzed pollen count data collected from 1993 to 2004 from a monitoring site in Atlanta.
They analyzed daily levels of pollen for five groups of plants: oaks, grasses, ragweed, pines, other evergreens, such as juniper and cypress, and another group of trees including birch and hazelnut.
Darrow's group compared the daily levels of pollen to the number of emergency department visits at 41 hospitals in the greater Atlanta area.
During the study period there were more than 400,000 ER visits related to asthma or wheeze.
The researchers found that days when oak and grass pollen levels were elevated, there were more ER visits.
For instance, days that were in the 95th percentile or above for grass pollen levels were tied to a nearly 10 percent increased risk of ER visits, compared to days that had average or below levels of pollen.
Days with such high pollen counts occurred about six times a year, and a 10 percent increase in trips to the ER meant about 16 extra visitors per day.
Similarly, days in the 95th percentile for oak pollen levels were tied to a nearly 15 percent increased risk in visits to the ER.
Dr. Sunit Jariwala, an assistant professor at Yeshiva University's Albert Einstein College of Medicine in New York, said that given the more than 25 million asthmatics in the U.S., a 10 to 15 percent increased risk of going to the hospital is important.
"Just from the sheer scale of things it's quite significant," said Jariwala, who was not involved in the study.
ER visits vary widely in cost, but are at least several hundred dollars.
NO EFFECT FOR RAGWEED?
Surprisingly, the researchers did not find any link between ragweed - which causes allergies for many people - and increased ER visits.
Darrow said the plant might still have an influence on asthmatics' symptoms, but "it was difficult to distinguish between ragweed and the back-to-school effect of asthma."
Asthma symptoms tend to ramp up when kids return to the classroom and there's a rise in colds and other viruses that can exacerbate the condition, she told Reuters Health.
The authors note in their study in The Journal of Allergy and Clinical Immunology that the effect of pollen on asthma might change as climate shifts alter pollen production.
Darrow said it's not clear yet what those changes might be.
One study has found that the ragweed season is already longer than it was nearly two decades ago. (See Reuters report of February 21, 2011: reut.rs/hlQBbv.)
As for the impact of current pollen levels, Jariwala said Darrow's results are likely to be an underestimate of the full impact of pollen on asthma symptoms, because the researchers only tallied emergency department visits.
"A fraction (of people affected) are actually seen" in the ER, he told Reuters Health.
Some people might suffer through it without seeking medical help, and others might go their family physician instead.
"There's all different levels of severity," agreed Darrow. "It's only the tip of the iceberg."
SOURCE: bit.ly/QeoWQ8 The Journal of Allergy and Clinical Immunology, online July 30, 2012.