Poisonous encounters: Nitrates in drinking water

A study finds that long term exposure to high levels of nitrates in drinking water can lead to health effects such as shorter height or stunting.

Nitrogen pollution of water can lead to severe consequences not only for the environment, but also to human health. Current evidence shows that nitrogen pollution of water is on the rise not only in developing, but also in developed countries.

A World Bank Policy Research Working Paper titled 'The Nitrogen Legacy: The long-term effects of water pollution on human capital' highlights the grave challenge of nitrogen pollution of water and the health risks arising out of drinking nitrate contaminated water. The report presents the findings of a study that evaluates the long term impacts of nitrogen polluted drinking water on health by analysing health and water pollution data from three countries namely India, Vietnam and Africa.

Water pollution due to nitrogen in India

Water pollution due to nitrogen is assuming alarming proportions in India. The green revolution that started in the mid 1960s in India not only led to a rapid increase in agricultural productivity, but also led to a dramatic rise in the consumption of synthetic nitrogenous fertilisers such as nitrogen phosphate potassium (NPK). NPK fertiliser use has shown a five fold increase per hectare of cultivated land since the mid 60s and this has led to major changes in the nitrogen cycle.  For example, India’s water runoff from the fields shows increased concentrations of nitrogen beyond unsafe levels. Besides this, sewage and organic solid wastes are one of the fast growing sources of nitrogen in the country.

In water, excess nitrogen promotes the growth of algae and triggers toxic blooms that can kill fish while nitrate in drinking water can greatly harm human health. Health impacts of nitrogen pollution can be acute, causing infant death due to methemoglobinemia, or the blue baby syndrome that reduces the blood’s ability to transport oxygen. Studies also show that early‐life exposure to nitrogen‐related pollution can lead to infant mortality and damage to health in the form of low birthweight and shorter height in adulthood. Low birthweight is a well known factor that can lead to many health problems during adulthood like coronary heart disease, decreased glucose tolerance and increased rates of mortality.

Health impacts of nitrogen polluted drinking water

However, evidence on the long‐term impacts of early‐life exposure to nitrogen pollution on health continues to be limited. This study addresses this gap by examining the impact of nitrate‐nitrogen pollution exposure experienced during early stages of life and its connections to health in later life by looking demographic and health data from India, Vietnam and Africa.

For India, the study combined district level data from the National Family Health Survey (2015-16) with water quality data from monitoring stations across 375 rivers in India between 1963 and 2017. The study observed upstream and downstream river flows to determine the direction of flow of pollutants, and tracked pollution exposure at the district-level for women between their births and the age of three.

The study found that:

Nitrogen exposure had a significant health impact on women

About 3 percent of the women in the sample had experienced high levels of nitrate‐nitrogen pollution in water (exceeding 10 mg/l) in the year of birth and on an average, women were exposed to high levels of nitrate nitrogen pollution for 2 percent of their lives upto the age of three.

Women exposed to nitrate pollution exceeding safe limits (10mg/l) during their early childhood were shorter (by 2.2 cm) than women not exposed to it.

For downstream districts, where faecal matter accompanied upstream nitrates, the effects were worse, with the height difference increasing to 3 cm. Analysis across different geographical settings across Vietnam and Africa also showed similar results.

The study results show that early life exposure to nitrogen pollution can greatly influence health outcomes with decreases in height observed across different life stages such as adulthood in India, childhood in Vietnam and infancy in Africa.

There are a number of ways in which nitrate levels influence health outcomes. For example, dietary intake of nitrate has been associated with hypothyroidism and thyroid cancer. The thyroid gland is very important for regulating hormone production and metabolism regulation. Hypothyroidism can thus lead to stunting of growth and delay in the process of maturation. Nitrates also lead to buildup of algae and bacteria in water. These bacteria emit cytotoxins that are toxic to humans leading to diarrhoea and repeated diarrhoeal infections often lead to nutritional deficiencies and stunted growth. This stunted development can not only limit the full potential of an individual as an important part of the society, but also has economic costs.

The study estimated a 1.7 percent loss in earning potential for people affected because of pollution exposure.

Health impacts were significant even at low nitrogen levels

The study found that health effects of nitrogen pollution also manifested themselves at pollutions levels that were well below prescribed limits thus raising questions about the currently set safe standards for nitrates in drinking water. For example, in India, the study found that the damaging health effects persisted even when pollution levels were below the government’s prescribed safety limits for Indian rivers.

The paper argues that it is important to question and rethink on the existing safety limits set for nitrogen pollution of drinking water in India.

The paper also questions the appropriateness of fertiliser subsidies in developing countries such as India where a system of domestic price controls by way of large subsidies has distorted market prices for nitrogen fertiliser resulting in excessive fertiliser application and highlights the need to improve nitrogen use efficiency in agriculture.

The paper can be accessed here

Note: This paper is a product of the Water Global Practice. It is part of a larger effort by the World Bank to provide open access to its research and make a contribution to development policy discussions around the world.

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