Air Pollution and Mortality in the Elderly in Kerman, Iran

Today, air pollution is one of the major environmental issues in the world’s major cities and is jeopardizing the living conditions of human beings (1, 2). High population growth and increased energy consumption, rapid economic and urban development, expansion of urban traffic, inappropriate use of industrial systems and devices, and non-compliance with environmental regulations has resulted in air pollution in many countries (3).

According to the World Health Organization (WHO), approximately 3 million people lose their lives annually due to air pollution (4). WHO has estimated that exposure to particulate matter causes more than 500,000 premature deaths per year. The numerous adverse events that have occurred as a result of air pollution in the world can appropriately highlight the importance of this issue. In October 1948, sulfur dioxide (SO₂) and industrial dust caused the death of 20 people and sickened almost 6,000 people in Pennsylvania. In London in December 1952, burning coal in homes and SO₂ emissions along with inversion and air pollution entrapment caused the death of 4000 people. In December 1962, again air pollution in London took the lives of 300 people (3).

It seems air pollution has a very comprehensive impact on human health. The severe effects of carbon monoxide (CO) on human health include increased mortality due to heart and respiratory diseases, stroke, and acute heart attacks. Studies have shown a relation between respiratory diseases and environmental pollution in industrialized countries as well as countries with low or moderate income. Therefore, air pollution is nowadays a major global environmental health problem (5).

One of the groups that are likely to be more susceptible to air pollution is the elderly. A previous analysis of an elderly cohort in Hong Kong revealed that long-term exposure to PM2.5 and black carbon was associated with cardiovascular mortality, but not respiratory mortality, among an elderly population in Hong Kong, a high-density and high-rise city in Asia, and effect estimates remained similar for various time exposure windows (6). Yap et al. (7) conducted a time series study in Singapore and revealed that particulate air pollutants (PM_2.5, PM₁₀) were associated with non-accidental mortality and cardiovascular mortality, and the effects were greater on the elderly. Another study in Montreal showed positive associations between daily non-accidental mortality and all air pollutants, except ozone. They also noticed that people with cardiovascular disease, congestive heart failure, atrial fibrillation, diabetes, and cardiovascular disease are more likely to die from air pollution (8).

This study aimed to investigate the relation between air pollution and elderly death in Kerman, Iran. This was the first study of this kind done in Kerman. Kerman City is one of the major cities of Iran located in the southeast, with a population of over 621,374 individuals according to the 2011 census (9).

This was an ecological study conducted in Kerman city. Initially, data on the number of deaths in the elderly per day, from 2006 until 2013 were inquired from the Deputy of Health of Kerman University of Medical Sciences. Elderly deaths were deaths that occurred due to any reason in people aged 60 and older. Then, these deaths were classified based on gender (male and female) and the cause of death (cardiovascular disease, respiratory diseases, trauma, diabetes, and other diseases).

Data on air pollution (including NO, CO, NO₂, NO_x, PM₁₀, SO₂, and O₃) were inquired from the Environmental Protection Agency of Kerman Province. These data are measured on a daily basis using a fixed station within the city. In this study, observations that were more than ± 3 SD away from the mean were recognized as outliers and were dealt with as missing data. Then missing air pollution data were estimated using the expectation-maximization (EM) algorithm in SPSS20 software. Details of these methods can be found in our previous publications (10). This study was approved by the Standing Committee on Ethics in Research of Kerman University of Medical Sciences (ethics code: IR.KMU.REC.1394.428). The relation between daily mortality and air pollution was analyzed using negative binomial regression in STATA13.

During this time, 6465, 2958, 1017, 895, and 3458 deaths caused by cardiovascular diseases, respiratory diseases, trauma, diabetes, and other reasons, respectively, occurred among the elderly in Kerman. The numbers of deaths have been categorized on the basis of year and gender in Table 1. The average air pollutants during these seven years are shown in Table 2. The relation between daily levels of air pollutants and deaths in the elderly caused by cardiovascular diseases, respiratory diseases, trauma, diabetes, other diseases as well as the total number of diseases; categorized based on gender have been shown in Tables 3 to 8. The results of multivariate analysis of air pollutants indicated that nitrogen oxide (NO) was directly and significantly related to total numbers of deaths in the elderly; and an increase in SO₂, ozone, and NO was significantly related to increased mortality among elderly women (Table 3).

The results of multivariate analysis indicated that CO was directly related to cardiovascular death of the elderly, but this relation disappeared in the gender subgroups, probably because the population size decreased (Table 4). Air pollutants did not show any significant effect on respiratory- (Table 5), trauma- (Table 6), and diabetes- (Table 7) related deaths in the elderly of Kerman city. The results of multivariate analysis of air pollutants indicated that ozone played a significant role in increasing the death of the elderly who died from other diseases (Table 8).

This study aimed to investigate the relation between air pollutants and the number of elderly deaths in one of the major Iranian cities. The results indicated that CO played a significant role in the cardiovascular deaths of the elderly. Indeed, CO triggers complex pro-inflammatory phenomena in the airways and combines with blood hemoglobin, and forms a stable hybrid, which is carboxyhemoglobin, thereby causing hypoxia and heart failure. Moreover, it affects the central nervous system and eventually, can lead to death (11). Studies conducted in Taiwan also showed a relation between CO and cardiac mortality, especially in the elderly and in the winter (12). Another study conducted on British individuals aged over 45 years, also found a significant relation between CO and cardiovascular deaths (13). In Tehran, studies found that the most important effect of CO was on the cardiovascular system and increase in cardiovascular deaths (14), and there was a significant relation between CO and the daily number of deaths among people aged over 64 years in Tehran (15). The present study also showed that CO could probably affect deaths among the elderly. However, we found no significant relation between CO and deaths caused by respiratory diseases, trauma, or diabetes in the elderly.

In this study, there was a relation between NO and total deaths in the elderly, especially women. However, the results of this study did not show a significant impact of nitrogen oxides on deaths caused by cardiovascular diseases, respiratory diseases, trauma, diabetes, or other diseases in the elderly. The results of a study on people aged 50 - 64 years in Denmark showed no significant relation between nitrogen dioxide (NO₂) and deaths caused by cardiac diseases, including myocardial infarction, angina pectoris, high blood pressure, and stroke and diabetes (16). However, the results of a study from Canada on people with an average age of 60 indicated a relation between all causes of death and NO₂ (17). Wong in China also showed a relation between NO₂ and deaths caused by cardiac diseases among hospital-admitted patients (18). Given the aforementioned controversies and inconsistencies, it seems that NO₂ and its association with mortality require further investigation.

The results of this study showed that SO₂ did not have a significant impact on deaths caused by respiratory diseases, trauma, and diabetes, as well as other diseases in the elderly. However, there was a significant relation between SO₂ and all deaths among elderly women. Interestingly, the aforementioned issue was not seen in elderly men. In a study conducted in Kerman, Iran increased SO₂ was significantly related to respiratory deaths in men (19) and was also associated with increased respiratory hospital admissions among women (20). However, in a study conducted in Beijing long-term exposure to SO₂ was not related to respiratory deaths (21). Similarly, in another study from Taiwan, there was no relation between SO₂ and respiratory deaths (12), and in Spain, there was no relation between SO₂ concentration and patients admitted to hospitals due to asthma attacks (22) either. This lack of relation was also reported in another study conducted in New York (23).

In the present study, there was a significant relation between ozone and total deaths in elderly women as well as total deaths caused by other diseases. However, the results of multivariate analysis indicated that ozone did not significantly correlate with elderly deaths caused by cardiovascular and respiratory diseases as well as trauma and diabetes. Another study from China indicated that there was no significant relation between ozone and deaths caused by respiratory diseases (18) either. Also, ozone concentration was not related to asthma attacks in Spain (22). In contrast, ozone was related to respiratory deaths in men in Kerman (24).

Apparently, PM₁₀ did not increase the death of elderly people in Kerman, Therefore, more research should be conducted in this regard. Several mechanisms have been proposed about how PM may impact death, including pneumonia, exacerbation of atherosclerosis, changes in heart function, inflammation of the air sacs, aggravation of lung diseases, increased blood clotting, increased blood viscosity, increased plasma fibrinogen, and changes in heartbeat (25).

However, it seems that further studies are still needed to determine the mechanism of its action. Inhalation of PM can cause oxidative stress and inflammatory responses in the lungs (26). Also, PM can penetrate into the lungs in the form of dust. Besides, they may pass through the lungs and enter blood circulation and reach other organs (27). In studies conducted in Taiwan, increased systolic and diastolic blood pressure was associated with increased annual average levels of PM_2.5 and PM₁₀ (28). In another study conducted in China, a relation was seen between PM₁₀ and the number of overall deaths, and the impact was more severe on people aged 45 to 65 years than younger people (25). Conversely, another study conducted in Taiwan found no significant relation between PM₁₀ and respiratory deaths (12). Furthermore, a significant relation was seen between short-term changes in deaths and PM₁₀ and PM_2.5 in several US cities (29). In a previous study in Kerman, PM₁₀ significantly increased respiratory mortality among men (24).

Epidemiological studies conducted over the past two decades around the world have shown that the effects of air pollution on human health and deaths associated with air pollution are on the rise (30).

Kerman City encounters sand storms in the spring and autumn (31). Also, several stone, sand, and gravel crushing plants, as well as asphalt plants, operate around the city, which contribute to this city’s air pollution. Therefore, it seems essential to pay particular attention to vulnerable groups, particularly the elderly on days with high air pollution. Suitable planning such as reducing elders’ commuting time on high air pollution days can help prevent mortality. Table 9 presents the results of several studies conducted in various regions around the world about the effect of air pollution on mortality in the elderly. Similar to the findings of our study, some other studies also suggest that air pollution can contribute to increased cardiovascular and respiratory mortality in the elderly.