Abstract
Background
Although both particulate matter with a diameter of 2.5 μm (PM 2.5) and black carbon are associated with cardiovascular disease, data on the correlation of PM 2.5 and black carbon with acute ischaemic stroke, particularly based on ecological research, remain limited.
Aim
To evaluate the association of PM 2.5 and black carbon with cases of acute ischaemic stroke in Thailand adjusted for physical factors.
Methods
In our ecological study, we collected data from the Health Insurance Database, which covers approximately 70% of Thailand’s population from 77 provinces, about numbers of patients with acute ischaemic stroke who were admitted and reimbursed. The data of PM 2.5 and black carbon were collected. The predictive model of cases of acute ischaemic stroke in relation to PM 2.5 and black carbon was computed by Poisson regression analysis adjusted for physical factors.
Results
During the study period, 201 023 patients were diagnosed as having acute ischaemic stroke in 77 provinces in Thailand. The median of PM 2.5 and black carbon of all provinces was 29.19 (range: 18.88–34.50) and 1.17 (range: 0.43–2.29) µg/m3, respectively. PM 2.5 and black carbon were significantly associated with cases of acute ischaemic stroke with adjusted coefficients of 0.008 and 0.179, respectively (p<0.001 for both factors).
Conclusions
Our ecological study showed that both PM 2.5 and black carbon are associated with cases of acute ischaemic stroke in Thailand.
Keywords: EPIDEMIOLOGY, STROKE
WHAT IS ALREADY KNOWN ON THIS TOPIC
Particulate matter with a diameter of 2.5 μm (PM 2.5) has been shown to be related to acute ischaemic stroke in North America and Europe but not in Asia.
Black carbon may also be associated with acute ischaemic stroke.
There are limited data on the association of those two air pollution particles on the numbers of patients with acute ischaemic stroke.
WHAT THIS STUDY ADDS
Higher levels of PM 2.5 and black carbon correlate with an increased number of acute ischaemic stroke.
Low temperature, high wind speed and low income showed a significant association with the number of acute ischaemic stroke cases.
HOW THIS STUDY MIGHT AFFECT RESEARCH, PRACTICE OR POLICY
National policies may be needed to control the levels of PM 2.5 and black carbon as lower levels may reduce the numbers of patients with acute ischaemic stroke.
Introduction
Acute ischaemic stroke is a common condition that accounts for up to 87% of all strokes.1 The global incidence of acute ischaemic stroke in 2017 was 101.3 per 100 000 people (95% CI 91 to 113.6). However, in some countries, the incidence may be higher; China, for example, has an incidence of 238.8 per 100 000 people.1 Acute ischaemic stroke has several risk factors, including age, sex, diabetes, hypertension, sleep apnoea and dyslipidaemia.2 3 Air pollution, especially fine particulate matter (PM) with a diameter of 2.5 μm or less (PM 2.5), is another risk factor of stroke.4 Unlike PM 10, PM 2.5 is a smaller particle associated with acute ischaemic stroke.5 A meta-analysis has shown that PM 10 presented a non-significant risk for stroke (95% CI 0.999 to 1.005) but that PM 2.5 had a significant OR of 1.006 (95% CI 1.002 to 1.010).5 Even though the effect size of PM 2.5 was small or 0.6% of increasing risk, this effect for large populations may be highly meaningful. However, another systematic review has revealed that the correlation between PM 2.5 and stroke may be region specific, given significant results found in North America and Europe but not in Asia.6 Those findings may imply that further studies are required on the topic, particularly in Asia.
Black carbon, one component of PM 2.5, is a fine particulate constituent primarily from traffic, industry and residential heating.7 It has been shown that black carbon is associated with cardiovascular disease with an HR of 1.294 (95% CI 1.158 to 1.446).8 However, data regarding the association between black carbon and ischaemic stroke remain controversial. While a positive correlation between black carbon and stroke was not found in at least two studies,8 9 it did emerge in another study.10 In addition to both air pollution particles, physical factors, including temperature or wind, have been reported to be associated with acute ischaemic stroke.11,14 Cold weather of −2 °C and strong wind over 14 m/s increased numbers of stroke admissions.14 Most studies have evaluated the association of those two air pollution particles on an individual basis, and some studies did not adjust for physical factors. In our study, we aimed to evaluate the association of PM 2.5 and black carbon on numbers of patients with acute ischaemic stroke by adjusting for physical factors.
Materials and methods
In our ecological study, we collected data from the Universal Coverage Scheme Database about patients of acute ischaemic stroke who were admitted and reimbursed between 1 January 2014 and 31 December 2016. This insurance is basic government insurance, which covers approximately 70% of Thailand’s population. Diagnosis of acute ischaemic stroke was made and retrieved based on the codes of the International Classification of Disease, tenth edition, for cerebral infarction (I63). Clinical data were also collected regarding age (ie, by age group), sex and comorbidities (eg, hypertension, diabetes, dyslipidaemia, atrial fibrillation, coronary artery disease, chronic kidney disease and previous stroke). The data collected represented numbers of patients with each factor by province. The income and population of each province were also collected.
Data about PM 2.5 and black carbon were obtained from the Modern-Era Retrospective Analysis for Research and Applications, V.2, generated by NASA’s Global Modelling and Assimilation Office: https://gmao.gsfc.nasa.gov/gmao-products/merra-2/. Data of temperature and wind speed in each province were retrieved from the Meteorological Department of Thailand.
Statistical analyses
Descriptive statistics were used to summarise features of each factor by province and reported as the median number of patients from all provinces. Data concerning PM 2.5, black carbon and cases of acute ischaemic stroke in each province (online supplemental table) were recorded and categorised in six geographical regions: north, northeast, west, central, east and south. We analysed the relationship between PM2.5, black carbon and the number of acute ischaemic stroke cases using Poisson regression. Studied factors were evaluated as potential predictors in univariate Poisson regression analysis. Factors with a p value less than 0.05 according to univariable Poisson regression analysis were included in the subsequent multivariate Poisson regression analysis, which was performed to identify predictors for numbers of acute ischaemic stroke. Results are presented as coefficients, 95% CIs and p values. Statistical analyses were performed in Stata (College Station, TX, USA).
Results
During the period studied, 201 023 patients were diagnosed with acute ischaemic stroke in 77 provinces in Thailand. The age group of 61–70 years had the highest median numbers of acute ischaemic stroke patients at 613 patients per province, while males had higher median numbers of acute ischaemic stroke patients than females (1175 vs 954 patients per province). Hypertension was the most common comorbid disease with a median of 956 persons. The median income and population were 8.67×103 baht/month and 639 957 persons, respectively (table 1). The median values for PM 2.5 and black carbon in all provinces were 29.19 (range: 18.88–34.50) and 1.17 (range: 0.43–2.29) µg/m3. The medians of black carbon over 3 years were quite steady at 1.24 (range: 0.41–2.37), 1.08 (range: 0.50–2.29) and 1.15 (range: 0.37–2.21) µg/m3, respectively. WHO global air quality guidelines recommended in 2021 that the highest average annual emission level of PM 2.5 was 5 μg/m3, while there was no recommended level for black carbon. The median temperature and wind were 27.06 °C (range: 23.41–28.43) and 4.29 knot/m (range: 2.68–5.23). Those data represent the medians of all provinces.
Table 1. Median and range of numbers of studied variables of all provinces in Thailand.
| Factors | Median number of patients (range) |
|---|---|
| Age, years | |
| <15 | 2 (0–49) |
| 15–45 | 133 (17–1310) |
| 46–60 | 545 (81–4834) |
| 61–70 | 613 (82–4331) |
| 71–80 | 534 (71–3299) |
| >80 | 246 (56–1983) |
| Male | 1175 (190–8505) |
| Female | 954 (117–6761) |
| Hypertension | 956 (137–9503) |
| Diabetes | 590 (46–5342) |
| Dyslipidaemia | 424 (35–3701) |
| Atrial fibrillation | 109 (14–934) |
| Coronary artery disease | 60 (5–435) |
| Chronic kidney disease | 96 (12–551) |
| Previous stroke | 94 (10–873) |
| Population | 639 957 (184 207–5 690 775) |
Data regarding average PM 2.5, black carbon and cases of acute ischaemic stroke by province and region appear in table 2. The northeast region had the highest average PM 2.5 at 31.03 μg/m3, which was higher than the national average of 27.76 μg/m3. For black carbon, the central region had the highest level at 1.58 μg/m3, which was higher than the national average of 1.15 μg/m3. The northeast region also had the highest average number of patients with acute stroke at 3361.95 individuals, which was higher than the national average of 2610.68.
Table 2. Mean (SD) black particle (μg/m3), PM 2.5 particle (μg/m3), number of patients with acute stroke and number of population by regions of Thailand.
| Regions | Mean black carbon | Mean PM 2.5 | No. of acute stroke | Population |
|---|---|---|---|---|
| North | 0.98 (0.10) | 27.60 (2.82) | 2036.44 (1277.37) | 696 074.96 (477 760.94) |
| Northeast | 1.24 (0.19) | 31.76 (1.49) | 3361.95 (2633.23) | 1 094 529.22 (609 127.17) |
| West | 1.13 (0.40) | 24.53 (1.58) | 1887.60 (647.07) | 667 821.06 (186 617.04) |
| Central | 1.58 (0.29) | 30.03 (1.43) | 2829.40 (2915.57) | 915 864.60 (1 115 505.75) |
| East | 0.89 (0.24) | 27.07 (2.32) | 2552.42 (1243.69) | 661 141.56 (380 704.73) |
| South | 0.57 (0.09) | 20.05 (0.87) | 1850.28 (1509.69) | 661 948.36 (411 248.37) |
| Total | 1.15 (0.42) | 27.76 (4.47) | 2610.68 (2265.11) | 851 151.33 (731 165.79) |
PM 2.5, particulate matter with a diameter of 2.5 μm.
PM 2.5 and black carbon were significantly associated with cases of acute ischaemic stroke with unadjusted coefficients of 0.038 and 0.341, respectively (tables 3 and 4). In the multivariate model, both PM 2.5 and black carbon were still independently related to cases of acute ischaemic stroke with adjusted coefficients of 0.005 and 0.136, respectively (p<0.001 for both factors), as shown in tables 3 and 4. In the black carbon model, temperature and income were inversely associated with numbers of acute ischaemic stroke with adjusted coefficients of −0.141 (p<0.001) and −0.019 (p<0.001), while wind speed was positively associated with cases of acute ischaemic stroke with an adjusted coefficient of 0.414 (p<0.001) (table 4).
Table 3. A predictive model of numbers of patients with acute ischaemic stroke by PM 2.5 using Poisson regression analysis.
| Factors | Unadjusted coefficient (95% CI) |
Adjusted coefficient (95% CI) |
|---|---|---|
| PM 2.5 | 0.038 (0.037 to 0.039)* |
0.005 (0.004 to 0.006)* |
| Temperature | 0.092 (0.088 to 0.096)* |
−0.126 (−0.131 to −0.120)* |
| Wind | 0.385 (0.378 to 0.392)* |
0.462 (0.451 to 0.471)* |
| Income | −0.018 (−0.019 to −0.018)* |
−0.020 (−0.021 to −0.196)* |
Indicating p value <0.001.
PM 2.5, particulate matter with a diameter of 2.5 μm.
Table 4. A predictive model of numbers of patients with acute ischaemic stroke by black carbon particle using Poisson regression analysis.
| Factors | Unadjusted coefficient (95% CI) |
Adjusted coefficient (95% CI) |
|---|---|---|
| Black carbon | 0.341 (0.330 to 0.351)* |
0.136 (0.124 to 0.148)* |
| Temperature | 0.092 (0.088 to 0.096)* |
−0.141 (−0.146 to −0.135)* |
| Wind | 0.385 (0.378 to 0.392)* |
0.464 (0.455 to 0.472)* |
| Income | −0.018 (−0.019 to −0.018)* |
−0.019 (−0.020 to −0.019) |
Indicating p value <0.001.
Discussion
Our ecological study showed that both PM 2.5 and black carbon are significantly associated with the number of patients with acute ischaemic stroke.
Several studies have shown that PM 2.5 is a risk factor for acute ischaemic stroke.15,19 The multiple mechanisms that may explain the correlation between PM 2.5 and ischaemic stroke include increased systemic inflammation, oxidative stress, endothelial damage, thrombosis via coagulation defect, the progression of atherosclerosis and vulnerability to plaques.20 For black carbon, a study from Sweden has shown that the particle, particularly from traffic, increased the risk of stroke by 4.1% per 0.31 mg/m3 of black carbon,21 which another study from Sweden has corroborated.22 However, the outcomes of both Swedish studies are not specific to acute ischaemic stroke as in our study. Both Swedish studies had the outcomes for both haemorrhagic and ischaemic stroke. Another study from Spain showed that black carbon was marginally associated with the incidence of ischaemic stroke by 1.05 times (95% CI 1.00 to 1.10).10 These results indicated that black carbon may be a potential risk factor for ischaemic stroke. Black carbon was also reported to be associated with higher mortality in cases of cardiovascular disease, including stroke.23,25 Those findings may indicate that black carbon is associated with the incidence and severity of acute ischaemic stroke.
Three factors are associated with the number of patients with acute stroke: temperature, wind and income. Temperature and income were inversely associated with the number of patients with acute stroke, whereas wind speed was positively associated (tables 3 and 4). Those findings were similar to the results of another case-crossover study from Germany.12 Regarding the association between income and acute ischaemic stroke, a study on data from the UK Biobank showed that the second-highest income group had the lowest risk for ischaemic stroke, with an HR of 0.701 (95% CI 0.618 to 0.795).26 Several studies have also supported the association between the rate of strong winds and increased numbers of stroke and acute ischaemic stroke.13 14 A study from South Korea showed that a high rate of wind speed had an OR of 1.20 (95% CI 1.08 to 1.34) for acute ischaemic stroke.13
The strengths of our study include that it examined complete data of all provinces under the Universal Coverage Health Security Insurance Scheme Database in Thailand for 3 years and that the model was adjusted for physical factors. However, there are some limitations to our findings. Personal factors reported, such as comorbidities, were not included in the model; instead, those data were collected as numbers of patients. The diagnosis of acute ischaemic stroke was also made according to the administrative dataset, so its misdiagnosis may represent a bias. Data used in the model represented the average of 3 years; however, the data about black carbon were quite stable across the 3-year period. Some factors, such as population data, were also not included in the multivariate model because they made the model uninterpretable. Only temperature and wind were included in the model as these two physical factors were reported to be associated with acute ischaemic stroke. The predictive model indicated numbers of patients with acute ischaemic stroke, not the disease’s incidence. Finally, the study population in this study included 70% of the country population; those with government insurance or social security insurance were not included.
In conclusion, both PM 2.5 and black carbon were associated with acute ischaemic stroke in our ecological study. Lower levels of PM 2.5 and black carbon may reduce numbers of patients with acute ischaemic stroke. National policy may be needed to control the levels of PM 2.5 and black carbon.
Supplementary material
Acknowledgements
This study was supported by Research and Graduate Studies, Khon Kaen University and North-Eastern Stroke Research Group, Khon Kaen University, Thailand.
Footnotes
Funding: The authors have not declared a specific grant for this research from any funding agency in the public, commercial or not-for-profit sectors.
Provenance and peer review: Part of a Topic Collection; Not commissioned; internally peer reviewed.
Patient consent for publication: Not applicable.
Ethics approval: Not applicable.
Data availability statement
Data are available on reasonable request.
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Associated Data
This section collects any data citations, data availability statements, or supplementary materials included in this article.
Supplementary Materials
Data Availability Statement
Data are available on reasonable request.