Asthma seasons around the world are unique to each individual: the time, place, and cause varies for each person.

By Bryan Ellis, MA, RRT

There are many different types of seasons. There is a cold and flu season. There are the seasons of spring, summer, fall, and winter. For popular music lovers, there are the “Season of the Witch” and “Seasons in the Sun.” And for some, there is the asthma season. Depending on where persons with asthma live, work, play, and go to school, their season of asthma will vary. It will vary depending on the allergen or activity that triggers an exacerbation, and it will vary depending on geographical location. It will vary depending on environmental temperature activity, humidity fluctuations, barometric pressure changes, levels of air pollutants, and aero-allergen concentrations. The relationship between asthma and environmental factors appears to be very complex, with the final result being that the asthma season is unique to each individual.

The relationship between environmental factors is so complex that apparently there are asthmatics who are sensitive to thunderstorms. The association between thunderstorms and subsequent exacerbations of asthma in susceptible individuals is just a correlation, not cause and effect. In London, a thunderstorm occurred and instead of the usual 2.25 asthma presentations per day, the emergency department at St Mary’s Hospital noted 40 asthma presentations in a 24-hour period following the storm.1 The subsequent retrospective study of the asthma epidemic by the hospital, located in west central London, identified two primary asthma classifications for the study period: epidemic asthma and nonepidemic asthma.

The nonepidemic asthma presentations were those identified as cases accounting for the daily average during the 2-month study period. The analysis demonstrated weather factors, air pollution, and grass pollens as predictors of asthma presentations. Analysis of the epidemic asthma associated each of the following with a significant increase in asthma presentations: number of lightning strikes, increase in rainfall, temperature drop or rise, and increase or decrease in air pressure and relative humidity. As with nonepidemic asthma presentations, concentrations of grass pollens correlated with an increase in epidemic asthma presentations. Although air pollution factors (vehicle exhaust pollutants) were associated with nonepidemic asthma presentations, they were not independently associated with epidemic asthma presentations.

In contrast to the St Mary’s Hospital study, another study from Finland2 found that nitrogen dioxide (NO2) as an air pollutant was associated with increased asthma presentations, but pollen levels did not correlate with asthma attacks. Of weather factors considered, it did find temperature to have an association with asthma, but not air pressure, rainfall, or humidity.

A separate study in Israel3 examined the relationship between emergency department visits for asthma and air pollutants, weather conditions, and airborne allergens. The study demonstrated a strong correlation between asthma presentations and air concentrations of nitrogen oxides (NOx, where x is 1, 2, or 3), sulfur dioxide (SO2), and barometric pressure. A negative correlation was observed between asthma attacks and ozone (O3) concentrations. Also noted in this study is a very high peak of asthma presentations during the month of September, which coincided with the beginning of the school year and the Jewish holidays. The authors suggested an increase in viral infections and emotional stress as factors accounting for the high number of emergency department visits.

Another recent study based in Mexico City4 examined the relationship between aeroallergens, air pollution, and weather, and emergency department asthma admissions. The airborne concentrations of pollens and fungal spores were monitored as were air concentrations of O3, NO2, SO2. Additionally, weather factors were monitored and a dichotomous classification of wet season (May through October) and dry season (November through April) was defined.

Overall, the study found that asthma admissions demonstrated a seasonal pattern with more admissions during the wet season. The study also found that air pollutants did not correlate with emergency department admissions for asthma in the age groups studied (children under 15, adults, and seniors) during either the wet or dry season. In contrast, it found an association, during both wet and dry seasons, between child and adult admissions and levels of grass pollens. During both seasons, child admissions correlated independently with fungal spore concentrations.

Although the study conducted in Mexico City did not find a relationship between air pollutants and emergency department admissions for asthma, this was not the case in Spain. This study5 took place in Valencia, and examined the association between daily asthma admissions to an emergency department and the air concentrations of O3, NO2, SO2, and particulate matter. The results of this study, in contrast to the Mexico City findings, were that a significant association between O3 and NO2 air concentrations and asthma admissions was present. The association between SO2 and particulate matter concentrations and asthma admissions was less significant.

In reviewing literature dealing with asthma and seasonal variations in air pollutants, aeroallergens, and meteorological factors, a reference to dust mites was found.6 Normally, dust mite-related allergy is considered to be a year-round problem and not seasonal. The hypothesis that allergic asthmatic patients present with seasonal exacerbations of bronchial hyperreactivity is consistently supported by the literature. These changes in bronchial hyperreactivity parallel exposure to the allergens specific to the season. Yet, in addition to seasonal allergens, there is evidence that some mite-sensitive patients also present with seasonal exacerbations of their bronchial hyperreactivity due to the effects of temperature and humidity on dust-mite populations. With a rise and fall in dust-mite populations due to weather factors, asthmatics are also potentially susceptible to periodic exacerbations of their asthma.

In this examination of recent literature relating to asthma and seasons, it becomes clear that there certainly is neither a singular season nor a singular cause of asthma attacks. From studies conducted in Finland, England, Israel, Spain, and Mexico, it appears that the asthma season is as wide-ranging and complex as the geography and weather patterns of these countries. Not only do fluctuating concentrations of allergens (pollens, fungal spores, dust mites) and air pollutants (ozone, nitrogen oxides, SO2, particulate matter) play a role, but the weather itself (rainfall, barometric pressure, temperature, humidity) seems to play a role both as a trigger and as a factor in the activity of previously mentioned triggers (aeroallergens, air pollutants).

For the asthmatic, it seems prudent to monitor patterns of asthma attacks year-round with an attempt to recognize a relationship between the attacks and environmental factors. With this knowledge, the patient can move from a reactive role of treating an asthma attack to a management and prevention program. Working with physicians for medical intervention, and taking advantage of informational resources that can be found in health care libraries and through associations such as the American Lung Association, can lead to a proactive relationship between the person with asthma, the environmental factors that trigger periodic exacerbations of bronchial hyperreactivity, and the health care team.

Most important, knowing the time and place where asthma attacks are most likely to be triggered by environmental factors will enable asthmatics to better define their season. With this knowledge, seasonal asthmatics can get a better handle on the prevention and management of their asthma.


Bryan Ellis, MA, RRT, is the clinical coordinator of the Adult Critical Care Respiratory Care Services Department at California Pacific Medical Center, San Francisco. For further information contact [email protected]


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