Meteorological changes and air pollution have been a public health concern for the last several decades due to accelerated global warming and desertification. Contemporary studies document the effect of temperature and air pollution on human morbidities, especially cardiovascular, respiratory and neurological systems, as well as on human mortality, even within the normal range of temperatures. For every rise of 1°C temperature, there is an increased cardiovascular mortality by 3.44% (95% CI 3.10–3.78), respiratory mortality by 3.60% (3.18–4.02), and cerebrovascular mortality by 1.40% (0.06–2.75) according to a 2016 epidemiological meta-analysis of 4 million participants for mortality and 12 million for morbidities (Aditi et al., 2016). Furthermore With 1°C rise in temperature, the incidence of many diseases increased, including diabetes mellitus, infections, and other morbidities of the cardiovascular, respiratory, and genitourinary systems (Aditi et al., 2016).
Meteorological changes have also been linked to ocular disease and Augera et al. (2017) determined an association between elevated outdoor temperatures and an increased risk of traction retinal detachment. Hu et al. (2007) found that primary angle closure glaucoma admission rates were significantly higher with increased relative humidity, but with no correlation to temperature. Matthew et al. (2016) reported a higher frequency of infectious keratitis during the higher temperatures and humidity levels of summer. Christoph et al. (2016) supported a correlation between higher weekly average temperature and increased ophthalmology emergency room visits.
To the best of our knowledge few studies have investigated the effect of meteorological changes on conjunctivitis. Of the studies examining conjunctivitis and seasons, they negate to account for isolated and direct effect of temperature on the conjunctivitis incidence (Chiang et a., 2012; Hong et al., 2016; Szyszkkowicz et al., 2016).
Conjunctivitis significantly impacts health care systems and incurs financial burdens worldwide. In the United States, 4–6 million conjunctivitis visits annually have entailed nearly 800 million dollars in treatment costs (Schneider, 2014). The direct effects of the condition on patients’ quality of life can vary, ranging from lost school/work to irreversible eye and vision damage. With the financial and quality life impacts, more information about environmental risk factors are crucial to develop measures to reduce incidence and burden on public health (Chen et al., 2019).
The pathophysiological mechanisms of air pollution and metrological changes on conjunctiva remain to be characterized. Some studies (Li et al., 2017; Gao et al., 2016) have speculated that PM2.5 and PM10 particles cause intraocular epidermal cells to lose their ability to adapt, leading to cell death and inflammation. Krishna et al. (1996) pointed to the strong oxidative stress effect of NO2 and O3 that may stimulate conjunctival cell inflammation. While various authors have related subtypes of conjunctivitis to specific seasons – viral conjunctivitis common in summer, bacterial in winter, allergic in spring – there is no consensus among them on the exact mechanism (Azari 2013; Chiang et a., 2012; Hong et al., 2016; Szyszkkowicz et al., 2016 ).
We found no association between air pollution (PM2.5, PM10) and the incidence of conjunctivitis (Table 2 in the Appendix). In the Negev Desert, there was low level of anthropogenic (chemical) pollution, supporting that non-anthropogenic air pollution is not related to conjunctivitis. However, other studies may have had chemical pollution confound the effect of PM on the incidence of conjunctivitis.
Our findings of seasonal differences in the incidence of conjunctivitis agree with Hong and co-workers (2016) where higher levels of temperature and lower humidity lead to increased outpatient visits for allergic conjunctivitis, potentially the result of pollen production in warmer temperatures. In our study, we investigated the meteorological effect on non-specific conjunctivitis while this study targeted allergic conjunctivitis. Furthermore, our study measured exact temperatures in relation to each patient’s condition while Hong et al. grouped patients by season. Chiang et al. (2012) found a peak incidence of chronic conjunctivitis in summer, mainly in rural areas and less prominent in urban areas, a difference they attributed to factors such as socioeconomic status, income and occupation. Szyszkowicz et al. (2016) found the number of visits to the emergency room due to conjunctivitis was higher in warm seasons (58%) compared to cold seasons (42%), as seen in our study with the incidence of conjunctivitis in the summer (36.4%) (P<0.001) and winter (26%) (P<0.001). Of note, these authors studied the effect by season and not, as we did, by the effect of temperature.
The statistically significant higher incidence of conjunctivitis in summer and lower incidence in winter suggests that higher temperatures are risk for conjunctivitis and lower temperatures are protective against conjunctivitis. We determined that certain temperature ranges are associated with incidence of conjunctivitis in summer and autumn. However, the lack of association between temperature and conjunctivitis in spring (moderate climate), and winter (cold climate) indicates a multifactorial relationship with other factors involved in conjunctivitis. Further research is needed to understand more of these factors.