How does the local environment impact hypertension risk?
Hypertension forms part of metabolic syndrome, which is a cluster of conditions that also includes excess body fat around the waist, high blood sugar levels, and abnormal levels of cholesterol or triglyceride in the blood.
The above are also risk factors for hypertension, as are smoking, dietary factors, such as high salt intake, drinking too much alcohol, and stress.
Because both hypertension and metabolic syndrome affect a growing number of people, understanding the range of factors that leads to these conditions is vital.
Some researchers are investigating the potential impact of where we live. In this vein, scientists from the Lithuanian University of Health Sciences and Vytautas Magnus University, also in Lithuania, recently published new findings in the Journal of Public Health.
Air pollution and hypertension
Earlier studies investigating exposure to air pollution and its relationship with hypertension produced conflicting results. However, a meta-analysis of 17 studies published in the journal Hypertension in 2016 concluded:
"Our results suggest that short-term or long-term exposure to some air pollutants may increase the risk of hypertension."
The authors of the latest study, which uses data from Kaunas, in Lithuania, paid particular attention to average exposure to ambient air pollution and the distance to green spaces and major roads. They also examined differences between living in multifamily homes, such as blocks of flats, and private single-family homes.
Specifically, they looked for links between these factors and the risk of developing arterial hypertension and certain measures of metabolic syndrome: reduced levels of high-density lipoprotein cholesterol (HDL, or "good," cholesterol), high triglyceride levels, obesity, and elevated blood sugar.
The study utilized data from three questionnaires taken by a total of 1,354 individuals; all of these participants had lived at the same location throughout the 10-year duration of the study.
The questions covered factors such as education level, alcohol consumption, smoking status, level of physical activity, blood pressure medication, and lipid-lowering treatment.
By using each participant's address, the scientists could predict their exposure to pollution. They also calculated the distance to the nearest green space, which they defined as a park larger than 1 hectare (10,000 square meters), and proximity to major roads.
The researchers also controlled for a number of variables, including body mass index, salt consumption, and education level.
All things considered, they found that long-term exposure to air pollution levels that were above the median increased the risk of having lower HDL. Higher than average exposure to pollution also increased the risk of having higher levels of triglycerides.
They also concluded that living closer than 200 meters to a major road increased the risk of hypertension.
Multifamily living and increased risk
Importantly, the scientists found that the impact of traffic-related exposure to air pollution was only significant for those who lived in multifamily homes. For individuals residing in single-family homes, their risk for hypertension did not increase, even if they were exposed to the same level of pollution as those in multifamily homes.
The authors believe that this is most likely because of other factors, aside from pollution, that go hand in hand with living in these types of complexes. For instance, living in relatively cramped conditions in a built-up environment might play an independent role in increasing risk.
On the other side of the coin, the researchers found a positive effect of living near public green spaces. The authors write that "The risk of the incidence of [arterial hypertension] was higher for persons living further than 300 meters from a [green space]."
"Our research results enable us to say that we should regulate as much as possible the living space for one person in multifamily houses, improve the noise insulation of apartments, and promote the development of green spaces in multifamily houses."
Lead author Agne Braziene
The authors' conclusions are interesting, and the results add weight to similar previous findings, but this topic is notoriously difficult to study for a number of reasons.
For instance, people who live in multifamily homes are more likely to have a lower overall income; earlier research has shown a relationship between socioeconomic status, metabolic syndrome, and coronary heart disease risk.
The researchers also explain that from the start of the study people in multifamily homes were significantly more likely to have diabetes and low HDL cholesterol than individuals in single-family homes.
Also, it is impossible to ascertain exact levels of exposure to noise and pollution for each participant. Someone who spends a great deal of time at home will have very different exposure levels from her neighbor who commutes a long distance for work, for instance.
Although the researchers attempted to control for some of these factors, it is not possible to remove their influence entirely.
That said, the evidence is mounting. Exactly how much impact air pollution and proximity to traffic have on our health is yet to be defined, but it seems increasingly likely that it is having at least some negative impact.