How does the season of birth influence the risk of asthma and allergic rhinitis?

By Dr. Chinta SidharthanReviewed by Benedette Cuffari, M.Sc.Aug 20 2024

In a recent letter written to the editor of Clinical and Translational Allergy, Finnish researchers report that season of birth likely impacts an individual’s future risk of developing airway allergy and related diseases.

Study: Being born in autumn or winter is associated with asthma and allergic rhinitis in Finland. Image Credit: Halfpoint / Shutterstock.com

Does your season of birth affect disease risk?

In countries that experience intense variations in sunlight exposure throughout the year, the likelihood of certain non-communicable diseases is believed to be associated with the season of birth. In fact, one recent study reported the existence of an association between adult-onset asthma and season of birth in Finland.

Nevertheless, these studies have produced mixed results. Although some studies have reported that being born in the summer and fall seasons is associated with an increased risk of allergic rhinitis, other studies have reported an association between being born in the summer and spring months and a higher likelihood of developing asthma.

About the study

Researchers conducted a broader examination of the association between the season of birth and allergies of the respiratory system, including allergic rhinitis, asthma, non-allergic rhinitis, nonsteroidal anti-inflammatory drug (NSAID) exacerbated respiratory disease (N-ERD), and chronic rhinitis with and without nasal polyps.

Follow-up data were obtained from Finland’s health registry for patients who had visited hospitals in the districts of Uusimaa and Helsinki and were diagnosed with respiratory and otorhinolaryngological conditions. Various demographic factors including age, gender, and date of birth were extracted from the health records. Several keyword searches were also performed using International Statistical Classification of Diseases Tenth Revision (ICD-10) codes to extract information about respiratory allergic conditions.

The seasons of birth were divided into winter, spring, summer, and autumn or fall. Winter months were from December to February, March to May were spring months, whereas summer and autumn were from June to August, and September to November, respectively.

Study findings

The incidence of asthma in individuals born in summer, autumn, winter, and spring months was 41.1%, 42.7%, 43.1%, and 42.1%, respectively. The incidence of allergic rhinitis was also lower among individuals born in the summer months, with the proportion of allergic rhinitis cases 10.7% in the summer as compared to 12.1%, 12.6%, and 12.0% for spring, winter, and autumn months, respectively.

The season of birth was not significantly associated with the incidence of chronic rhinosinusitis with or without nasal polyps or with N-ERD. The incidence of chronic rhinosinusitis among individuals born in the summer, autumn, winter, and spring months was 9%, 8.7%, 9.1%, and 9.5%, respectively. The proportion of individuals with N-ERD was also largely uniform across summer, autumn, winter, and spring seasons of birth at 1.3%, 1.5%, 1.5%, and 1.4%, respectively.

The occurrence of allergic rhinitis modified the association between season of birth and chronic rhinosinusitis; however, this effect was not significant. Autumn and winter seasons of birth showed a combined association with allergic and non-allergic rhinitis. Similarly, spring and winter seasons of birth were associated with an increased risk of both asthma and non-allergic rhinitis.

These patterns suggest that exposure to greenery during pregnancy might play a role in the increased risk of asthma and allergic rhinitis in the infant. However, the results also indicate an association between adult-onset asthma and birth during winter months. These contrasting observations could be related to the birth decade, which was identified as a contributing factor by previous studies.

The potential mechanisms through which the season of birth impacts the incidence of respiratory allergies and allergic conditions include seasonal changes in exposure to luminosity and changes in vitamin D metabolism, as well as exposure to nature and air pollutants that impact the immunological responses and increase the risk of allergic rhinitis.

Conclusions

The researchers’ findings suggest that season of birth could contribute to the incidence of allergic rhinitis and asthma, with exposure to luminosity, greenness, and air pollutants during pregnancy potentially involved in the development of allergic immunological responses.

No significant associations were observed between season of birth and chronic rhinosinusitis, N-ERD, or non-allergic rhinitis. Thus, more research using a larger and more diverse cohort is needed in the future.