Association of Asian Dust with daily medical consultations for pollinosis in Fukuoka City, Japan

Asian Dust (AD) is a meteorological event that affects Japan especially in the spring when dusts are transported eastward via airstreams from the deserts of China and Mongolia. A number of studies have suggested that AD can adversely impact human health. Specifically, earlier research has linked AD to an increased risk of all-cause [1‐4], cardiovascular [1, 5, 6], cerebrovascular [3] and respiratory mortality [7, 8]. Several studies have also indicated that the risk of hospitalization for respiratory [9, 10], cerebrovascular [11] and cardiovascular diseases [10, 12] may increase on AD days.

There is some evidence that AD may also contribute to the worsening of allergic disease symptoms [13‐15], although this association has not been consistently observed in previous studies [16‐18]. In Japan, severe AD events have been found to be associated with an increased risk of hospitalization for asthma [13] and aggravated lower respiratory symptoms [14]. On the other hand, a study using data from Fukuoka in Kyushu Japan showed that the odds of asthma hospitalization among children for a 10 μg/m³ increase in SPM did not differ between the AD and non-AD days, suggesting a lack of association between children asthma hospitalization and AD events [16]. Non-significant associations have also been reported in a study of daily asthmatic admissions in Taipei [17] and daily clinical consultations for conjunctivitis in the same location [18].

To our best knowledge, no previous study has examined if AD might potentially contribute to an increase in the number of medical consultations for pollinosis. This is an important research gap, especially since there have been a few small-size studies (n = 23 to 54) which indicated that allergic symptoms such as allergic rhinitis – a main symptom of pollinosis – might be aggravated by AD [19‐21]. For example, a cross-sectional study has suggested that AD might act as an adjuvant in promoting allergic rhinoconjunctivitis induced by the inhalation of allergens such as pollens and fungi [19]. Another study has reported that nasal and ocular allergic symptoms are associated with AD before and after the Japanese cedar pollen season, but not during [20]. The worsening of nasopharyngeal, ocular, respiratory, and skin allergic symptoms have also been observed on AD event days in a study in Yonago, a city facing the Sea of Japan [21].

The objective of the current study is to examine the association between airborne AD and the daily number of medical consultations for pollinosis. We used data from Fukuoka City, which is located in the Kyushu region of Japan close to the Eurasian continent with frequent experiences of AD events in the spring (Fig. 1). The associations of health problems, such as asthmatic symptoms and ischemic stroke with AD have been previously investigated in this location [16, 22]. In the past, some studies have relied on PM₁₀/PM_2.5 to ascertain the occurrence of AD [1, 17, 18, 23]. However, recently it has been proposed that AD may serve as an aid to enhance allergic reactions independent of particulate matter [13‐15, 19‐21]. Hence, in the present study, an AD event was defined based on the criteria set by the Japan Meteorological Agency, i.e., when any of the air monitoring stations located in Fukuoka City reported a turbid atmosphere, i.e., visibility < 10 km, or when AD particles are visible via remote sensing or sun photometer. We also included the suspended particulate matter (SPM) in the current study in view of its relationship with AD (i.e. higher SPM level on AD days) [16], and controlled for airborne pollen level as the main trigger of pollinosis in all analyses. Assuming that the exacerbation of pollinosis symptoms by atmospheric environment is reflected in the daily number of medical consultation for pollinosis [24], we analyzed data comprising 24 years of daily consultations for pollinosis and the environmental factors (i.e. AD and SPM) in Fukuoka City using time series regression.

AD was observed on 238 (6.6%) of the 3630 days included in the study. AD occurred with a median value of 7 days per year between February and April. A total of 65,488 clinic visits for pollinosis were analyzed. Across the 4 clinics, there were 9.2 visits per day during the 3-months period. The frequency of visits did not differ statistically between AD-affected and AD-free days (Table 1). The level of SPM was significantly higher when AD was present, while the concentration of pollen was only marginally higher. Temperature and relative humidity differed significantly based on the presence of AD.

AD was associated with increased risk of clinic visits for pollinosis at lag 0 (pooled RR = 1.175, 95% CI 1.061–1.301, Table 2). The estimated risk increased gradually until lag 4, with lags 3 to 5 being statistically significant before attenuation at lag 6. The 6-days cumulative RR (lags 0–5) was 1.215 (95% CI 1.031–1.431). In the bi-pollutant model, the estimated RR for the effect of AD increased slightly when SPM was added to the model. The estimated effects of SPM and its interaction with AD were small and generally insignificant (Table 2 and Additional file 1: Table S1).

To further examine the effect of SPM given the higher concentration during the AD events, we stratified the effect of SPM into AD-affected and AD-free days (Table 3). The estimated 6-days cumulative effect of SPM (lag 0–5) was slightly larger on AD-affected days than on AD-free days (RR = 1.066, 95% CI 1.007–1.127 vs. RR = 1.062, 95% CI 1.022–1.104), although the difference was not statistically significant. When Clinic IV was excluded because of the shorter study period, the difference remained statistically insignificant. The estimated effects of SPM were delayed for approximately 5 days and attenuated at lag 6 on both the AD-affected and AD-free days (Table 3). The breakdown by clinic is presented in Additional file 2: Table S2.

Findings of the current study suggest for the first time that AD is associated with an increased number of daily medical consultations for pollinosis in Japan. The number of consultations increased by approximately 21.5% (95%CI 3.1% – 43.1%) on average across the four clinics when there was an AD event (Table 2). The association was short-term beginning immediately at lag 0 (i.e. on the same day) and observed up to the previous 5 days. When SPM was included as adjustment, the estimates of AD effect remained significant (but they increased slightly likely because of collinearity). We did not find evidence of interaction between AD and SPM (Additional file 1: Table S1), despite the higher SPM level on AD days. The results of further analysis showed that SPM was associated with the daily consultations for pollinosis, with a lag structure that was fairly similar to that of AD (Table 3). The estimated effects of SPM were smaller than those for AD, and they did not differ between the AD and non-AD days, which has also been reported in a study of asthma admission among children and AD in the same location [16].

There are two possible ways AD might be linked to the exacerbation of pollinosis symptoms, which might in turn leads to an increase in the number of consultations. First, AD may act as an adjuvant for allergens [31]. AD particles contain soil-derived metals, anthropogenic metals, and other forms of atmospheric pollutants by human activity, and then act to promote nasal and ocular symptoms [31]. Second, AD components may contain bacteria, fungi and viruses, all of which can have a direct effect as allergens [3, 19, 32]. For example, earlier research has suggested that desert dust may contain fungi which are allergenic, [33], and that airborne bacteria may increase by approximately 4 times on AD days compared with non-AD days [32]. It has been suggested that these microorganisms may negatively affect the human immune system and exacerbate pollinosis symptoms [33, 34].

We observed a delay of less than a week (i.e. until lag 5) in the association between AD and the daily consultations for pollinosis in this study. This delayed effect was noted in all the clinics except Clinic II (Table 2). A possible explanation is that some patients were likely not able to visit the clinics on the same day when their symptoms became worse, and their visits later were registered as a lag effect [16]. It is also possible that the transported dust remains in the air for several days after an AD event even if it is not observed visually, although further investigation is required to understand this phenomenon, particularly the role of long-range fine/ultrafine particles that can remain suspended in the air for a longer period.

In this study, we examined the effect of AD defined based on meteorological conditions instead of the level of SPM. This allows us to examine the total effect of AD, which may encompass contribution by factors unrelated to SPM (e.g., microorganism-related AD factor described above). Our results showed that the effect estimates for AD remained significant after adjusting for SPM, implying there might be other air pollutants or unmeasured exposures that might have triggered or worsened the pollinosis symptoms on AD days. Besides the microscopic organisms, a likely explanation would be that AD might also represent smaller inhalable particles such as fine particulate matter (PM_2.5). It has been previously suggested that the aeroallergens from pollens might be able to bind to small air particles [35], and in Japan, a study has indicated that particles as fine as 0.3 μm might contain allergens such as Cry j 1 and Cry j 2 from the Japanese cedar pollens [36]. In addition, a recent study has indicated that the synergetic effect between airborne pollen and PM_2.5 was stronger and more robust than the interaction between pollen and SPM [24]. Nevertheless, because the measurements of PM_2.5 were not available for most of the study period (the data was only available since 2010), we were unable to examine this in the context of AD and pollinosis consultations. Future studies are therefore needed to explore the roles of air particles of different size, including their origin (e.g. transported or local), chemical constituents, and potential interactions with airborne pollen concentrations during the spring season.

This study has several limitations. First, we had no information on age, sex and individual exposure or about the participants’ pollinosis history or history of other allergic diseases. The association between the daily number of medical consultations and AD might differ across such individual characteristics. Second, it was not possible to separate the first clinic visits from re-visits in the same year. First visit refers to the first examination that occurs when a patient start to experience pollinosis symptoms in each pollen season, while re-visits are usually re-examinations for the purpose of medicine refill. In the present study, as data with separate first and re-visits were not available, it is possible the association between air pollen and clinic visits for pollinosis might have been underestimated, leading to incorrect estimates for AD and SPM. For example, two previous studies that used data from a clinic in Tokyo reported that the positive correlation between the pollen count and the number of first examinations for pollinosis was stronger than that for re-examinations [34, 37], suggesting that the first visits might better reflect the severity of pollinosis symptoms. Third, this study did not consider other criteria air pollutants such as sulfur dioxide, nitrogen dioxide and ozone, which require further investigations as there have been evidence linking air pollution and the prevalence of allergic rhinitis [38, 39].

AD is positively associated with the daily number of medical consultations for pollinosis in the spring season. The increase in risk began on the day of AD and was attenuated within a week. The underlying mechanisms through which AD exacerbates the symptoms of pollinosis should be examined in future research.