Air Pollution and Hospital Admission for Epilepsy in Kerman, Iran

Epilepsy is one of the most common neurological diseases and affects individuals of all ages (1). Air pollution might cause central nervous system (CNS) damage and neurodegenerative disorders (2). The occurrence of seizures might be affected by the interaction of internal and pathologic factors and extrinsic factors, including medication and environmental factors (3, 4). Studies have shown that Parkinson’s disease, Alzheimer’s disease, multiple sclerosis, and stroke might be related to ambient air pollution (5, 6). Calderon et al. conducted a study in Mexico and reported that exposure to air pollutants could seriously affect children’s CNS (7). Another study in China also showed the possible relation between air pollution and neurodegenerative diseases (8). Fluegge and Fluegge reported hospitalization for epilepsy associated with changes in the concentrations of various pollutants, including nitrogen dioxide (NO₂), carbon monoxide (CO), sulfur dioxide (SO₂), ozone (O₃), and particulate matter (PM). Additionally, Fluegge and Fluegge showed that air pollutants might be a risk factor for epilepsy hospital admissions (9).

However, the effect of air pollution on the CNS has not been thoroughly studied. Kerman has a population of about 740,000 individuals according to the 2016 census and is located in Kerman province in southeastern Iran (10). In addition to human-made air pollution, Kerman faces sandstorms and increased ambient dust in specific seasons. The prevalence of epilepsy in Kerman was 7.87 per 1000 individuals in 2011 (1). A recent review estimated that there are about 840,000 individuals with active epilepsy living in Iran, and epilepsy prevalence in Iran is 1% (11).

The present study aimed to evaluate hospital admissions in Kerman for epilepsy and its possible relation with air pollution.

In this ecological study, the concentrations of ambient air pollutants (i.e., CO, O₃, NO₂, SO₂, and particulate matter under 10 and 2.5 microns [PM₁₀ and PM_2.5]) were obtained from Kerman Environmental Protection Agency within September 2008 to March 2020. The meteorological data, including temperature and relative humidity, were obtained from Kerman Meteorological Organization. The aforementioned variables were adjusted as confounders.

The epilepsy hospitalization data were obtained from Kerman’s Shafa hospital epilepsy registry and according to the International Classification of Diseases 10th Revision, which was code G40. The data were investigated regarding gender and age subgroups (under 18, within 18-59, and over 59 years). The data of all epilepsy patients that had stayed in the hospital for more than one day were included.

In the less than 12-year period under study, 894 epilepsy admissions occurred in Kerman, 498 (55.7%) and 396 (44.3%) cases of whom were male and female, respectively. Table 1 shows the number of epilepsy admissions within 2008 to 2020 in different population subgroups. Table 2 shows descriptive statistics of daily air pollution and meteorological variables within 2008-2020. The findings showed that the mean daily concentrations of PM₁₀, PM_2.5, and SO₂ were higher than the World Health Organization daily thresholds reported as 50, 25, and 20 μg/m³, respectively (17).

Table 3 shows the results of the adjusted GAM about the effect of air pollutants on overall epilepsy admissions in male, female, and age groups adjusted for average daily relative humidity and temperature. Increased epilepsy admissions in the total population were observed in various lags for CO, NO₂, PM₁₀, and PM_2.5. Additionally, CO, NO₂, PM₁₀, and PM_2.5 had a direct association with epilepsy admissions in male subjects. The strongest relations were observed in lag 4 for CO (RR = 1.2352, 95% CI: 1.0298 - 1.4815), lag 0 for NO₂ (RR = 1.0409, 95% CI: 1.0282 - 1.0537), lag 2 for PM_2.5 (RR = 1.0066, 95% CI: 1.0014 - 1.0118), and lag 0 for PM₁₀ (RR = 1.0057, 95% CI: 1.0039 - 1.0075). PM₁₀ and PM_2.5 also had a direct association with epilepsy admissions in female subjects. The strongest relations were observed in lag 2 for PM_2.5 (RR = 1.0071, 95% CI: 1.0015 - 1.0128) and lag 6 for PM₁₀ (RR = 1.0031, 95% CI: 1.0011 - 1.0050).

In all age groups, CO, NO₂, PM₁₀, and PM_2.5 showed direct relations with epilepsy admissions. The strongest relation between CO and epilepsy admissions was observed in the over 59-year group in lag 0 (RR = 2.1455, 95% CI: 1.5823 - 2.9091), NO₂ in the over 59-year group in lag 0 (RR = 1.0407, 95% CI: 1.0139 - 1.0681), PM_2.5 in the over 59-year group in lag 5 (RR = 1.0157, 95% CI: 1.0062 - 1.0252), and PM₁₀ in the under 18-year group in lag 2 (RR = 1.0064, 95% CI: 1.0029 - 1.0098).

This study showed significant relations between short-term exposure to air pollutants CO, NO₂, PM₁₀, and PM_2.5 with epilepsy admissions in Kerman, Iran. In this study, CO increased epilepsy admissions. Consistent with the results of this study, Bao et al.’s study in China showed an association between CO and increased epilepsy hospitalization (1.1%, 95% CI: 0.1 - 2.1%) (18). In addition, in Mexico, there was an association between ambient CO and epilepsy admissions (RR = 1.098, 95% CI: 1.045 - 1.155) (19).

In this study, NO₂ had a significant relation with epilepsy admissions in total and several different age groups, and the strongest relation was observed in male subjects. Automobile exhaust is one of the most important sources of NO₂. Wang et al.’s study in China demonstrated a significant association between the NO₂ of automobile exhaust and neurobehavioral function in school-age children. In the aforementioned study, two primary schools were chosen. One school was located in a clear area and the other in a traffic dense and polluted area. NO₂ had been monitored for the effect of traffic-related air pollution on the school campuses and classrooms. Children participated in assisted neurobehavioral testing to assess neurobehavioral performance (20).

A systematic review in 2017 concluded that high concentrations of NO₂ in polluted air significantly affect the CNS in children and adults and represent a significant risk factor for human health (21). A study in China showed a significant relation between the increasing concentration of NO₂ with epilepsy attacks (2%, 95% CI: 0.5 - 3.6%) (18). Furthermore, in Mexico, an ecological study showed a significant relation between NO₂ and epilepsy attacks (RR = 1.083, 95% CI: 1.038 - 1.13) (19). In a cohort study in Denmark, residential exposure to road traffic and air pollution was associated with a higher risk for febrile seizures (IRR = 1.05, 95% CI: 1.02 - 1.07) (22). A study in southern Spain showed that even low levels of NO₂ exposure and traffic-related air pollution had adverse effects on children’s neurodevelopment (23). Xu et al.’s study in China showed that the RR for epilepsy attacks was 3.17 (95% CI: 1.41 - 4.93) per 10 μg/m³ increase of NO₂ (13). However, a study in the USA showed a protective effect for N₂O on epilepsy (IRR = 0.85, 95% CI: 0.74 - 0.97) (9). N₂O is a different compound and is derived mainly from agricultural fertilizers and natural sources; nonetheless, NO₂ is mainly produced by vehicles.

Another pollutant evaluated in this study was PM₁₀, which had a significant relation with epilepsy admissions in total and several subgroups, with the strongest relation observed in the under 18-year subgroup. Several studies have shown relations between ambient PM₁₀ and epilepsy attacks (19, 24, 25). Consistent with the results of this study, Cakmak et al. in Mexico showed an association between PM₁₀ and hospital admissions for epilepsy (RR = 1.083, 95% CI: 1.038 - 1.13) (19). A study in six cities in Italy demonstrated positive associations between PM₁₀ exposure and total emergency calls within 2002 to 2006 (26). Additionally, increased emergency calls for epilepsy attacks were observed with exposure to PM₁₀ in China (RR = 1.5, 95% CI: 1.1 - 2.0) (24). Radmanesh et al.’s study in Iran showed that patients with different types of headaches and epilepsy increased on dusty days, compared to clean days, and there were significant associations between increased concentrations of ambient PM₁₀ and hospital admissions for these problems (25). Another study from Iran showed that exposure to PM₁₀ increased oxidative stress and the expression of inducible nitric oxide synthase messenger ribonucleic acid levels and reduced the concentrations of antioxidant enzymes (27).

In this study, PM_2.5 had a significant relation with epilepsy admissions in total, and several subgroups, with the strongest relation, observed in the over 59-year subgroup. Consistent with the results of this study, a significant association was observed between PM_2.5 and epilepsy attacks in Mexico (RR = 1.065, 95% CI: 1.002 - 1.132) (19). Other studies have shown that oxidative stress, neuroinflammation, glial activation, and cerebrovascular damage are the primary pathways for inducing brain pathology by air pollution (28). Oxidative stress, changes in autonomous function, and progression of atherosclerosis can be exacerbated by exposure to ambient PM (29).

In this study, O₃ and SO₂ were inversely related to epilepsy admissions. In an ecological study in China, Xu et al. also reported negative associations between ambient O₃ and epilepsy attacks (-0.84%, 95% Cl: -1.58 - 0.09%) (13). An interventional study showed that O₃ could be protective against pentylenetetrazole-induced epilepsy attacks (30). However, in another study conducted in Mexico, a significant direct association was observed between O₃ and hospital admissions for epilepsy attacks (RR = 1.100, 95% CI: 1.025 - 1.181) (19). In Fluegge and Fluegge’s study in the USA, no significant relation was observed between O₃ and epilepsy attacks (9). In the current study the average concentration of O₃ was 30.21±11.19 ppb, which is less that Xu et al.’s study (mean = 100 ± 63 ppb) (13) which showed a negative association, and Cakmak et al.’s study (mean = 93.26 ppb) (19). Further research is needed to clarify the effect of O₃ exposure on epilepsy.

Some of the strengths of this study included about 12-year data on air pollution and epilepsy admissions and the use of GAMs to adjust for nonlinear confounder variables. However, given the ecological nature of this study, the results cannot be easily inferred at the individual level.