We explored the associations of ambient exposure to total pollen, as well as tree, grass and weed pollen, with respiratory symptoms and use of rescue medication among a cohort of asthmatic children. Most studies of allergens and asthma symptoms have focused on indoor exposures. These studies of outdoor allergens have usually used measurements from monitoring stations.9,27-29
Our study expands on previous work by incorporating a modeling approach to estimate individual ambient pollen exposures in a prospective cohort design. Estimates of individual-level ambient exposures, based on each child's place of residence, allow for a more detailed description of spatial variability in pollen exposures.
Few studies have examined ambient allergen exposures and individual asthma symptoms.30-32
We are not aware of any previous study examining specific asthmatic respiratory events while providing individual estimates of ambient exposures. Strengths of this research include detailed data on daily respiratory symptoms and medication use. In addition, the population included only children with physician-diagnosed asthma, providing a study population that is pertinent from a risk-management and treatment perspective.
Our findings indicate that sensitive populations (i.e. maintenance-medication users sensitized to a particular allergen) experience elevated risk of any respiratory symptom, wheeze, shortness of breath, persistent cough and rescue medication use with exposures to weed pollen as low as 6-9 grains/m3
quintile). This level of exposure is defined as “low” by the National Allergy Bureau.33
Odds of experiencing asthmatic symptoms also tended to decline with the highest quintile of exposure, which may indicate some behavioral modification when pollen concentrations are very high. Such behavior modification has been observed with high ambient O3
The possibility of behavioral response to ambient pollen concentrations has not been explored, and we lacked information on the daily activity patterns of children necessary to access this. The exposure model is least accurate at the extremes of the pollen distribution, and the possibility of exposure misclassification is therefore a concern at the highest exposure category. However, misclassification is unlikely to explain a lower OR in the 5th
quintile of exposure compared with the 4th
We were unable to investigate the effect of tree pollen on sensitized children due to lack of measurement of IgE specific to tree pollens. However, there was more evidence for an association between tree pollen exposures and respiratory symptoms among users of maintenance medication than among non-users. This may be because the users of maintenance medication are at higher risk for respiratory symptoms in general. Asthma exacerbations have been reported with exposure to both weed and tree pollens.7-8,35
However, other studies have shown no association or negative associations.9
Inconsistent findings are likely due to differences in the types and composition of pollens present, study populations (including sensitization profiles), and study design, as well as limitations in the assessment of pollen exposures.
We also observed that grass pollen increased likelihood of all respiratory outcomes among non-users of maintenance medication (the less severe asthmatic children) who were also sensitized to grass pollen. Our results are consistent with associations between grass pollen and asthma exacerbations shown in previous studies.7,9,36
A possible explanation for associations among non-users and not among users of maintenance medication may be due to the timing of the grass pollen season. Grass pollens are most prevalent in the summer. During this time children with severe asthma may be less likely to engage in rigorous outdoor activities compared with less severe asthmatics.37-38
Differences in lifestyle, including air conditioning use in the home, would also affect actual exposures of children. In addition, susceptibility to ambient air pollutants has been found to be lower among non-users of maintenance medication.10
There is also a greater proportion of Hispanic and black children within the non-user group. Race is a factor that has been associated with higher indoor allergen exposures,39
less frequent medication use and access to health care.40
Our findings are also pertinent to understanding future health burdens within the context of climate change. Increased pollen production has been associated with increased temperatures and CO2
Even a slight increase in pollen production could substantially increase the number of days with exposures high enough to elicit an asthmatic response among sensitive children, given our finding of increased likelihood of asthmatic response at low levels of exposure. Moreover, higher temperatures may also lead to greater allergenic potency (i.e. allergenicity, of pollen41
), which would further increase the risks associated with all levels of exposure, especially low levels. Although we did not observe any interactive effects between pollen and ozone, increasing air pollution levels may also exacerbate the effects of pollen on asthma symptoms.24-26
These circumstances would indicate that sensitive populations may experience even greater frequencies of asthma symptoms if current climate change predictions hold.
Classification of asthma severity is a challenge. Previous research with this cohort concluded that the categorization of severity by use of maintenance medication reflects individual vulnerability to the health consequences of air pollution.10
We classified asthma severity by use of maintenance medication, rather than GINA score or other combinations of symptom frequency and medication previously used to assess asthma severity,5-6,10,42-43
because our outcome measures were respiratory symptoms and rescue medication use.10
We observed that associations between specific pollen exposures and asthma symptoms differed by a child's asthma severity classification. Given these findings, further research investigating effect modification by asthma severity is warranted, including additional approaches of classifying severity.
This study focused on ambient pollen concentrations, which have been found to be higher than indoor levels.44
Additional research could also incorporate indoor pollen measurements. Ideally, a study of indoor and outdoor allergen exposures would include data on daily activity patterns since children spend the majority of their time indoors.3
Participation in outdoor activities may be even further limited among asthmatic children. Weather is also likely to influence children's activity and may be associated with ambient pollen concentrations. Additionally, home ventilation, use of air conditioning and other home characteristics may alter the relationship between indoor and outdoor pollen concentrations.
The results of this study indicate that risks of respiratory symptoms among asthmatic children with specific pollen sensitization are associated with pollen exposures even at low levels. A unique aspect and strength of this study is the assessment of individual ambient pollen exposures, and this approach to exposure assessment could be applied to other studies. These findings are further strengthened by the use of detailed records of daily respiratory symptoms and use of asthma medication, available on more than 99% of study days. Future studies incorporating indoor pollen exposures and activity patterns would be beneficial to further investigate the association between pollen exposures and asthmatic symptoms.