Every year when the kids go back to school after the summer break, asthmatic children experience a worsening of their symptoms that often results in hospitalization. This worsening of symptoms, or asthma exacerbations, cost the health care system $50 billion a year and cause millions of lost work and school days. In children, the seasonal variation in asthma exacerbations is remarkably consistent year to year and seems to mirror the return to school following an extended break. There are numerous holidays throughout the year where children are off school for a week or more at a time: Christmas, spring break, summer holiday. Asthma exacerbations spike so predictably after the summer holiday that it has been termed the “September epidemic of asthma”. An illustration of this effect can be seen in the image below. This epidemic has been seen in the US, Canada, and the UK.
Hospitalization rates for asthma symptoms in children. Credit: Eggo et al. PNAS 2016.
It has been theorized that poor air quality in schools, elevated pollution levels from roads around schools, and spreading of the common cold at schools could explain this spike in asthma exacerbations. Previous studies have focused on swabbing individual patients to identify viruses that may have contributed to the exacerbation and subsequent hospitalization. The problem with this method is that it is time consuming, limited in the number of people it can compare, and relies on a virus being detected in the patients in order to draw a conclusion. To get around this problem, a group of mathematicians built a complex computer simulation to compare viral infection rates for various common colds and the incidence of asthma exacerbations in 66,000 adults and children.
The researchers built a number of variables into their simulation that are thought to play a roll in this spike in asthma exacerbations following an extended absence from school. These included: incidences of common cold, ozone levels, and pollution levels. They found that above all other factors, an increase in the incidence of the common cold is the primary driver of the worsening of asthma symptoms after summer break. In addition, the rise and fall in the number of children infected with the common cold also followed the return to school after an extended break. It has been shown a number of times that viral infections (influenza, common cold) can trigger asthma exacerbations and hospitalizations. The theory is that on vacation, kids interact with other children less often and so their viral immunity decrease. When kids return to school, they are exposed to more viruses than they were at home which in turn triggers colds, and in asthmatic children, can trigger exacerbations. As any parent with a small child has likely seen, they consistently pick up colds from school or daycare. Therefore, it should come as no surprise that the spike in common colds follows the return to school after holidays. Interestingly, this paper saw that asthma exacerbations in adults followed the seasonal flu and not any rise and fall in common colds with the school schedule.
Header photo credit: Wellcome images Flickr
Science Translation 
The air problem should be the main one as so many students concentrate there and bad air ventilation will worsen the situation. Maybe the research focus can be relied on how to improve the air quality with smell less and harm less chemicals.
They looked at air quality in the paper but I was not as big a driver of asthma exacerbations or hospital visits as viral infections. I do agree with your statement about air ventilation specifically when it applies to schools near high traffic roads. There is compelling data out now showing the effect of diesel exhaust on asthma symptom control. Additionally, proper ventilation and filtration may be able to help control the spread of viruses in the initial return to school after a break.