In this study among elderly Finnish subjects, chronic rhinosinusitis and chronic cough were associated with the long-term mean winter precipitation in the home municipality, whereas wheezing with dyspnea and sleep apnea were not associated with any climatic indices. The results suggest different contributions of climate on different chronic respiratory symptoms.
Previous studies about climate and respiratory symptoms often focus on extreme, short-term climatic conditions [8]. A large survey in 32 countries showed that in humid, subtropical regions, anomalously wet conditions increase the risk of cough [23]. A Japanese study found the mean vapor pressure to be the most important environmental factor associated with acute exacerbation of nonallergic rhinitis [24]. Our findings suggest that even mild, but long-lasting or repetitive high precipitation may increase the risks of chronic rhinosinusitis and chronic cough.
In Nordic countries like Finland, much of the winter precipitation comes in the form of snow. Thus, a large proportion of water is in a solid state until spring, when it melts during a relative short period in March-April. At that stage, a large amount of water is released. This may be regarded as minor flooding, though it is a normal annual phenomenon. These minor annual floods may be, at least partly, responsible for the associations of chronic rhinosinusitis and chronic cough with high winter precipitation. Of note, the present survey was performed in April, in the middle of the melting period. This may have affected the results.
The association of high winter precipitation with the risks of chronic rhinosinusitis and chronic cough may mediate via an increase in the indoors humidity. People spend most of their time indoors and indoors absolute humidity correlates well with the outdoors absolute humidity throughout the year [17, 18]. High indoors humidity is well known to associate with cough and nasal symptoms. Under such conditions, the building occupants may be exposed to increased levels of microbial agents such as fungal spores, formaldehyde from building materials, as well as odors, microparticles and vapors [19–22]. However, just 1.4% of the subjects in the present survey reported about moisture damage in their home. Due to the low prevalence of moisture damage, its regional distribution could not be assessed in the present study.
Another explanation for the association between high precipitation and respiratory symptoms is that high precipitation may increase the pollen concentrations. The association between precipitation and the pollen load is complex. Rainfall during the flowering season decreases the pollen load whereas high precipitation during months preceding the pollination may increase it [25]. Winter precipitation probably precedes pollination and therefore, high winter precipitation could intensify the pollination. Unfortunately, we did not have information about local pollen counts for the present study, which can be regarded as a weakness.
It may be plausible to assume that the direct or indirect effects of excessive ambient moisture on respiratory symptoms mediate by altering the function of the airway epithelium. Chronic rhinosinusitis, chronic cough, and wheezing with dyspnea all may involve a dysfunctional airway epithelial barrier [26]. Therefore, it is interesting that only chronic rhinosinusitis and chronic cough were associated with high winter precipitation. We suggest three possible explanations. Firstly, the nose plays a primary role within the airways, working as a filter and air-conditioner [27]. Thus, it is probable that the effects of high indoors humidity or high pollen load affects more the nose than the lower airways [28]. Secondly, chronic rhinosinusitis is a well-known cause for chronic cough [12, 13]. Thirdly, wheezing with dyspnea is typical for asthma, which is a very strongly genetically determined disorder [29, 30]. Thus, environmental factors may play a minor role.
The recently updated climate change scenarios indicate that the winter precipitation may increase up to 40% during the current century in Finland, indicating almost 100 mm increase [4]. Our multivariate model suggest that this could increase the risk of chronic rhinosinusitis by 79% and the risk of chronic cough by 57%. These are alarming figures given the high prevalence and the high health resource utilization due to these symptoms [6]. However, projections to the future are complicated and our results should be interpreted with caution. For example, despite the increasing winter precipitation, the snow cover tends to get thinner in the Northern Hemisphere due to warming of the winters [31]. This probably decreases the amounts of water released during the melting period in spring. In addition, climate change probably leads to many other environmental phenomena. These include the intensification of pollen and other bioallergenic proteins production, including bacteria, viruses, animal dander, insects, molds, and plant species. Furthermore, climate change may also influence the ambient concentrations of air pollutants including carbon monoxide, lead, nitrogen dioxide, ozone, particulate matter, and sulfur dioxide. These variables could not be taken into account in the present study but are a capable to affect, and probably increase further, the risks of chronic rhinosinusitis and chronic cough in the future [8].
The present cross-sectional survey can only reveal associations, not causal relationships. Other weaknesses of the study include the limited generalizability of the results: They may only apply to countries with cool climate and an increase in precipitation due to climate change. In many other areas of the word, the climate change probably decreases precipitation [1]. In the present study, the associations between long-term temperatures and respiratory symptoms were weak or absent. This may be since people spend most of their time indoors and that in cool climates like that in Finland, indoors temperatures are kept rather stable throughout the year. In areas with warmer climate, the situation is different and the indoors air temperatures correlate better with the outdoors temperature [17, 18, 32, 33]. Furthermore, the present population was old, and the results may not be directly applicable to younger subjects. The participation rate was low, but the age and gender distribution of the responders closely resembled that of the target population.
The main strength of the present study is the accurate information about the long-term mean temperatures and precipitations in each subjects’ home municipality. The respiratory symptoms were strictly defined as currently suggested in the literature. Furthermore, many factors that could affect the risk of respiratory symptoms could be considered in the multivariate analyses: Smoking, socio-economical state, urban versus rural municipality, and the individual differences in experiencing and reporting of bodily sensations, which was covered by the variable ‘symptom sum’ [34, 35].
In conclusion, chronic rhinosinusitis and chronic cough showed strong associations with long-term winter precipitation in the home municipality. Given the anticipated rapid increase in winter precipitation in Northern America and Northern Europe due to climate change, these symptoms may become more prevalent and present an increasing burden on primary health care. However, more reliable projections for the future would require more comprehensive information about other climate change-associated phenomena.