Exposure to even moderate levels of arsenic in drinking water is associated with an increased risk of heart disease, especially among smokers, finds a study published on bmj.com yesterday.
Arsenic is a natural element of the Earth's crust and high concentrations in groundwater pose a public health threat to millions of people worldwide.
High levels of arsenic exposure from drinking water have already been related to an elevated risk of heart disease. Given the huge burden of heart disease worldwide, a small increased risk associated with moderate arsenic exposure could be of major public health importance.
In 2001, a report from the National Academy of Science (NAS) said that the Environmental Protection Agency (EPA) had underestimated the risk of bladder and lung cancer posed by arsenic in drinking water.
Several years ago, scientists speaking at the Royal Geographical Society (RGS) at an annual meeting in London, said this will lead to higher rates of cancer in the future. About 140 million people, mainly in developing countries, are being poisoned by arsenic in their drinking water, researchers believe.
"It's a global problem, present in 70 countries, probably more," said Peter Ravenscroft, a research associate in geography with Cambridge University.
"If you work on drinking water standards used in Europe and North America, then you see that about 140 million people around the world are above those levels and at risk."
South and East Asia account for more than half of the known cases globally.
So a team of researchers from USA and Bangladesh, led by Dr Yu Chen from New York University School of Medicine and Dr Habibul Ahsan from the University of Chicago, set out to test the association between arsenic exposure and death from cardiovascular disease and to assess whether cigarette smoking influences the association.
The study involved 11,746 men and women living in Araihazar, Bangladesh, where groundwater is contaminated by arsenic.
At the start of the study, participants were interviewed and underwent a physical examination. Urine samples were also taken and tested for levels of arsenic. This procedure was repeated at two year intervals for an average of 6.6 years.
Water samples were collected from 5,966 tube wells used by the study participants for drinking and arsenic levels were measured.
After adjusting for factors such as age, gender, smoking status and education level, the authors found a dose-response relation between arsenic exposure and deaths from heart disease at a lower level of arsenic exposure than previously reported.
The death rate for cardiovascular disease was 271 per 100,000 person years among individuals drinking water containing moderate levels of arsenic (12 - 864 parts per billion, or ppb) compared with 214 per 100,000 person years among individuals drinking water containing low levels of arsenic (less than 12ppb).
They estimate that almost 30% of these deaths in the study population can be attributable, in part, to moderate levels of arsenic concentrations in well water (12-864 ppb).
The risk of dying from heart disease associated with arsenic exposure was consistently higher in current and former smokers compared with never smokers, suggesting that the cardiovascular effects of arsenic exposure, even at moderate levels, are further intensified by smoking.
The ways in which arsenic leads to heart disease are not clear, but studies suggest that it can induce atherosclerosis (hardening of the arteries), say the authors. "Exposure to arsenic in drinking water is adversely associated with mortality from heart disease, especially among smokers," they conclude.
Arsenic poses far higher health risks than any other known environmental exposure, say Professors Allan Smith and Craig Steinmaus from the University of California, Berkeley, in an accompanying editorial. Yet water contaminated with arsenic is tasteless, looks crystal clear, and boiling the water only concentrates the arsenic in it.
They suggest that, in all parts of the world where groundwater is used for drinking, clinicians should ask patients about their drinking water and, if it comes from a well, urge them to have it tested for arsenic. "It is too late to identify exposure after diseases caused by arsenic have been diagnosed, because many are fatal," they conclude.