Pollution and Cardiovascular Health in Malta – A Review

Air pollution is a worldwide problem with almost all people exposed to some degree.¹ When discussing health care, the term air pollution encompasses gaseous pollutants present at ground level (ozone, carbon monoxide (CO), nitrous dioxide (NO₂ ) and sulfur dioxide (SO₂ )) as well as airborne particulate matter (PM), all of which are able to enter the body through the airways.  The latter is subdivided into three categories according to size of particles: coarse matter (<10μm - ≥2.5 μm), fine matter (<2.5 μm - ≥0.1μm) and ultrafine particles (<0.1 μm).³

Primary pollutants, such as soot particles, nitrous dioxide and sulphur dioxide, are emitted directly into the air by combustion of fossil fuels. Secondary pollutants are formed in the atmosphere from other components, such as ozone which is formed through complex photochemical reactions of nitrogen oxides and volatile organic components. Sources of coarse matter (PM₁₀ ) include dust, soil and dirt physically thrown into the air by wind or movement of vehicles. Once airborne, PM₁₀ may be inhaled and deposited into the throat and upper airways. On the other hand fine matter (PM_2.5 ) is mostly found in smoke and haze and is attributable to combustion processes taking place in human industry, power plants, motorized vehicles andresidential heating using oil, coal, or wood. PM_2.5 is of particular health concern as it is able to travel deeply into the respiratory tract and into the small airways.⁴  It is also important to highlight that PM_2.5 can travel over long distances (>100km) imposing its effect over a wider area.³

There is major variability in the levels of air pollution globally. As expected, lower-income countries have higher household pollution due to consumption of solid fuels for domestic heating and cooking. However this is also a problem in the west where despite spending most of their lives indoors, people are still exposed to PM_2.5 which has been shown to infiltrate buildings. More concerning is a shift to using ‘environmentally-friendly’ biomass fuels for heating over traditional electrical means, particularly prevalent in Northern Europe, creating major indoor air quality problems.³ Large urban-rural differences are found for soot (average 38% higher), NO₂  (63% higher), and ultrafine particle numbers. Temporal variation of daily air pollution concentrations is also present. For example, ozone concentrations are highest during the warmest, high-intensity sunlight hours of the day; traffic-related pollutants, such as ultrafine particles and soot, often peak during the morning and evening rush hours resulting in high exposures for people commuting. Wind direction, wind speed and atmospheric stability also effect daily pollution levels.

Air Pollution and Mortality

Cardiovascular Mortality

In 2004 the American Heart Association (AHA) first released a statement on their position of the effect on air pollution on health, concluding that exposure to particulate matter is indeed an adverse factor in cardiovascular morbidity and mortality.⁵ In a further review in 2010 the AHA consolidated their position by claiming exposure to PM_2.5 in the short term may trigger cardiovascular events but that there is an even greater effect in long-term exposure, reducing life expectancy by a few years.⁵ One meta-analysis found a pooled effect of a 6% increased risk per 10μg/m³ increase in PM_2.5 in all-cause mortality and as much as an 11% per 10μg/m³ PM_2.5 increased risk in cardiovascular mortality, which was higher when compared to death from non-malignant respiratory disease.⁶

In recent years, the European Study of Cohorts for Air Pollution Effects (ESCAPE study) was launched with an aim to investigate the effect of pollution on the health of >300,000 individuals across Europe. They used standardized models for measurements of exposure and subsequent health effects whilst trying to eliminate confounding factors such as race and social status that may exist between regions.⁷ ESCAPE revealed a statistically significant effect of PM_2.5 on all-cause mortality, with a non-significant association with cardiovascular mortality specifically. The effect was exerted with mean PM_2.5 levels lower than those currently recommended, a trend that will be found across many studies.⁸

Cardiovascular Disease

Cardiovascular effects of air pollution are now well-recognized and studies have increasingly shown pollution to both exacerbate existing heart conditions as well as predispose the individual to the development of cardiovascular disease including coronary artery disease, cerebrovascular disease and peripheral artery disease.³

Coronary artery disease

Long-term exposure to PM_2.5 was associated with an increased risk of coronary artery disease in previously healthy, post-menopausal women. In the Women’s Health Initiative Study, exposure to >65,000 women over 4 years in 36 different cities showed that risk of cardiovascular events was increased with increase in air pollution exposure. There was a hazard ratio of 1.21 for developing a first coronary event and a hazard ratio of 2.21 to die of the coronary event for every 10µg/m³ increase in PM_2.5 .⁹

The ESCAPE studyinvolvedmorethan 100 000 participants without a previous history of cardiovascular disease from 11 cohorts across Europe over the period 1997-2007.  It found a 12% increased risk of coronary events per 10 µg/m³ in PM₁₀ and a 13% increased risk of coronary events per 5 µg/m³ increase in PM_2.5. Most importantly, positive associations were also observed below the current recommended annual European limit for PM_2.5 and PM₁₀ .^10-11 Both of these studies found a higher risk in people over 60 years of age and non-smokers, with PM_2.5 exerting its effect at less than 25μg/m³ , the current legal limit recommended.These findings suggest pushing for the lowering of European limits of air pollution so as to better protect the public from cardiovascular disease.

Another study investigated the increased risk of myocardial ischemia with exposure to PM_2.5 in patients known to suffer from ischemic heart disease, through repeated exercise stress testing. There was an increased risk of myocardial ischemia (as defined by ST-segment depression on exercise stress testing) in those people who had been exposed to higher levels of PM_2.5 or the gaseous pollutants NO₂ and CO.¹²

More recently, a study involving six metropolitan areas in the United States, showed an association between air pollution and the development / progression of coronary artery calcium over 10 years, with a particular stronger association in people aged over 65, the hypertensive and non-obese. Also of interest was that the mean PM_2.5 level in these areas was lower and acceptable by international standards.²

Cerebrovascular disease

The Women’s Health Initiative study reported a 35% increased risk of stroke and death from cerebrovascular disease and 83% increased risk of death from cerebrovascular disease per 10μg/m³ increase in long-termPM2.5 exposure.⁹

The ESCAPE study found a 19% increased risk of stroke per 5 mg/m3 increase in PM2.5 in nearly 100 000 participants from 11 cohorts across Europe.¹⁰

Peripheral artery disease

Furthermore, air pollution and living closer to a main road was also been associated with peripheral artery disease as defined by worse ankle brachial indexes.¹³

Heart failure

While much research has suggested a link between coronary artery disease and air pollution, studies into its possible association with heart failure in the long-term are still unsubstantial. One meta-analysis included 35 studies from across North and South America, Europe and Asia looking into more than 4 million events. It linked an increased risk of hospitalization or mortality due to heart failure with increased levels of both CO (per 1ppm), SO₂ (per 10ppb), NO₂ (per 10ppb),PM_2.5 (per 10μg/m3) and PM₁₀ (per 10μg/m3), as much as 3.5%, 2.4%, 1.7%, 2.1% and 1.6% respectively.¹⁴

Arrhythmias and arrest

The effect on ventricular arrhythmias is unclear, with studies providing inconsistent evidence. While studies investigating a clear relationship between arrhythmias and pollution by measuring the rate of activation of implantable cardio-defibrillators have so far come up short, (15)other studies have reported an association between sudden cardiac death and out-of-hospital arrest with pollution.¹⁶  

Pathophysiology

Although recent data has shown a strong influence of air pollution on cardiovascular disease, the exact mechanism by which air pollution increases the risk of cardiovascular problems remains unclear. Several hypothesis of pathophysiology have been suggested.³

Vascular Dysfunction

One proposed mechanism is that exposure to air pollution predisposes to vascular dysfunction with an increase in vascular tone and thus blood pressure, contributing to long-term sequelae. A study discovered that short-term inhalation of PM_2.5 and ozone contributed to an acute rise in mean arterial blood pressure, secondary to acute arterial vasoconstriction. Subjects had been exposed to concentration levels comparable to the urban environment.¹⁷ Meanwhile larger observational studies have shown that exposure to PM_2.5 has led to persistent vascular dysfunction, and the lowering of PM_2.5 concentration exposure was associated with a drop in the mean blood pressure and a reduction in cardiovascular events.¹⁸

Atherosclerosis

Several studies have showed the association of formation of atherosclerosis with exposure to air pollution, in particular PM_2.5 .⁷ Using CIMT as marker for subclinical atherosclerosis, the Multi-Ethnic Study of Atherosclerosis (MESA) examined the progression of CIMT in comparison with PM_2.5 levels. Despite a mean follow-up of only 2.5 years, it showed a positive but not significant association between pollutionand CIMT and a greater reduction on PM_2.5 levels was associated with a slower rate of CIMT progression.¹⁹

A cross-sectional meta-analysis of data from studies, including the previously mentioned ESCAPE, showed a 0.78% increase in CIMT per 5μg/m³ increase in PM_2.5 as well as showing a positive association between CIMT and living proximity to high vehicular traffic.⁷

Increased Thrombogenicity

One hypothesis for the association of short-term exposure to PM_2.5 with acute cardiovascular events is the suggestion that prolonged exposure to air pollution results in endothelial damage and an increased likelihood of thrombosis through chronic inflammation and transient increases in plasma viscosity.²⁰

This effect seems to be particularly increased in patients who suffer from diabetes. A study confirmed this by investigating platelet function in adults with diabetes. The study concluded that a relative increase in PM_2.5 was associated with a greater prothrombotic tendency of platelets as well as an increase in blood leukocyte number that was used as a marker for inflammation.²¹

Air Quality in Malta

In recent years the European Union (EU) has recognized the negative impact that air pollution has on European health and economy and has set air quality limits which are regularly exceeded, especially in urban areas, the most troublesome pollutants being PM and ground-level NO₂ .²² Since joining the EU, Malta has committed itself to monitoring pollution levels and striving to reach targets agreed upon. The European Environmental Agency (EEA) monitors air pollution levels in all EU-28 member states through air quality data, which are submitted by each country to the EEA. The most recently published data encompasses the years of 2011-2015.²²

As depicted below in Figure 1, taken from of the EEA report published in 2017, Malta has managed to remain below the EU’s PM_2.5 limit of 25μg/m³ with a mean of 10-20μg/m³ . The main contribution of PM_2.5 in Malta is identified as road traffic and energy production, with road traffic accounting for approximately 6 times of emissions when compared to energy production in 2013. More worryingly, emissions from traffic are on the upward trend.²²

Using this data, the EEA has been able to quantify the effect of air pollution and express it as premature deaths. Malta, with a population of 425,000 people at the time of the study, was found to have an annual mean PM_2.5 of 12μg/m³ . By extrapolation this was calculated to contribute to 220 premature deaths annually.²²

The EU recommends annual mean levels of PM_2.5 and PM₁₀ of 25μg/m³ and 40μg/m³ respectively, however no standards have been set for the chemical composition of PM and no standards set for PM_0.1 levels, the so-called “ultrafine particles”.³

Although Malta reached PM_2.5 levels as recommended by the EU, it is important to also mention that in 2005 the World Health Organization (WHO) published global guidelines for air quality standards, in which the PM_2.5 limit was recommended at 10μg/m³ as an annual mean and the PM₁₀ limit was recommended at 20μg/m³ . Therefore at PM_2.5 of 12μg/m³ Malta exceeds this.²³ In fact it is estimated that one third of Europeans live in substandard air quality according to EU guidelines however as much as 90% are exposed harmful levels according to WHO criteria.³

A Timely Intervention

Since the harmful effects of air pollution have been clearly defined, the issue turns to the value of intervention on air quality control. A study from the United States investigated changes in life expectancy associated with changes in PM_2.5 levels during the 1980s and 1990s. A significant increase in mean life expectancy by 0.61 years was associated with a reduction of 10μg/m³ in PM_2.5 concentration, accounting for almost 15% of overall increase in life expectancy in areas studied.²⁴

Another study used the ban on coal sales in Dublin in 1990 as a pivotal point in examining air quality, investigating mortality rates over the 6 years prior and comparing to the following 6 years. Average black smoke concentrations fell by 35μg/m³ and, in conjunction, non-trauma mortality decreased by 5.7% and cardiovascular deaths fell significantly by 10.3%.²⁵

Strategy Propositions

In 2017 the Lancet Commission on Pollution and Health was published, which not only highlighted the effect of pollution on health but also proposed that global pollution can be controlled and pollution-related health problems prevented, advocating for several interventions. As Table 1 summarizes, such interventions are needed at population and individual level and as healthcare professionals we have a role to play too.

Pollution control strategies are dependent on a determined government and an engaged, informed and empowered civil society, with necessary collaboration between different government agencies and non-governmental organizations (NGO’s) both nationally and internationally. Successful intervention relies on primary prevention by eliminating pollution at the source, and an efficient system able to control pollution once present in the environment. Such proposals do not come into fruition overnight and therefore it is important that ambitious but attainable targets are devised, guided by both national and supranational standards such as WHO or the EU. For interventions to be effective they must have public support and be prioritized according to health effects, environmental damage and cost-effectiveness. Once implemented it is important to establish systems for monitoring the effects of such interventions and seek improvement in their significance.²⁶

Examples of high impact interventions to reduce air pollution are the energy and transport sectors. Highly relevant to Malta, the recent shift of energy production from coal power plants to cleaner gas plants while also incentivizing industry to move towards cleaner, more efficient production technologies is a step in the right direction being taken along with many developed countries. An even better strategy would be funding research and converting to low-polluting energy production such as wind and solar plants which would not only improve cardiovascular health of the population but also reduce greenhouse gases and increase the economic efficiency of energy generation.

Transportation also carries a heavy impact on air pollution, significantly so with the Maltese population who are so reliant on personal transportation. Upgrading public transportation to be more efficient and affordable for the populous as well as restricting motorized vehicles from city centres and encouraging active transport with the creation of appropriate walkways and cycle-lanes will significantly improve air-quality.²⁶ Moving toward electric cars rather than diesel / petrol will be an inevitable step in the future which should rather be done sooner than later. Paris have already targeted a ban on non-electric cars by 2030 and Copenhagen recently announced a plan for a total ban on diesel cars by 2019.

On an individual level, one can limit the effect of air pollution on health by personally reducing contribution to pollution through reduction of energy or transport use as well as the use of clean fuels in energy consumption. In preventing personal exposure to pollution an increased awareness of susceptibility to pollution is needed with subsequent behavioral change to reduce exposure.¹

Finally, as health professionals, we can contribute by emphasizing the relationship between pollution and adverse health effects, contributing research in environmental science, public health and health economics and supporting education in environmental health science in the younger generations.²⁶

Conclusion

In recent years, the problem of air pollution and its harmful effects have been highlighted with ample evidence published supporting the significant association of air pollution with risk of adverse cardiovascular effects. As a nation, Malta has elevated levels of air pollution and trends towards further worsening in recent years as industry and economy are booming. This is most likely resulting in significant morbidity and mortality due to cardiovascular disease to the Maltese people. We must now turn to solution through determination of the population including taking care of our own roles as individuals in society, by pressuring policy makers and supporting interventions carried out on a national scale so as to reverse the trend.

References

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