4.4 Nitrogen: a developing threat to health

A great deal of attention, by governments and the media, is focused on the environmental threat posed by carbon dioxide (CO₂) emissions and on the urgent need to reduce them. Mainly due to the burning of fossil fuels, the level of CO₂ in the atmosphere has increased by some 36% since pre-industrial times. According to recent estimates (IPCC, 2007), this increase has contributed more than 50% of the global warming attributed to human activities. The rest is due to enhanced atmospheric concentrations of a number of other ‘greenhouse’ gases, including nitrous oxide (N₂O). Molecule for molecule, N₂O is nearly 300 times more powerful as a greenhouse gas than CO₂ and levels of the gas are rising rapidly, largely due to the widespread use of nitrogen fertilisers in agriculture; some of the nitrogen ends up in the air as N₂O. However, the amount of N₂O in the atmosphere is still very low, and the observed increase (up 18% above pre-industrial level) has contributed just 5% to global warming so far.

Nitrogen exists in the environment in huge quantities; as nitrogen gas (N₂) it makes up 78% by volume of the Earth's atmosphere. As an element, nitrogen is an inert gas; that is, it is unreactive, is of little use to animals or plants and does no harm to them. It is when nitrogen becomes incorporated into compounds with other elements (notably oxygen or hydrogen) that it becomes reactive and can be harmful. In a natural environment, like that which existed before the industrial revolution, reactive compounds of nitrogen are produced naturally, in relatively small quantities. For example, lightning converts nitrogen in the atmosphere into compounds called nitrates that enter the soil in rain. Bacteria that occur in nodules on the roots of certain plants, known as nitrogen-fixing bacteria, also convert nitrogen gas into nitrogen compounds which enter the soil, providing a natural fertiliser.

The situation changed during the industrial revolution, with the burning of fossil fuels on a large scale, releasing various nitrogen compounds into the atmosphere in large quantities. More recently, the industrial production of nitrogen fertilisers has released into the environment a number of other, reactive nitrogen compounds. These are listed, with their effects, in Table 4 in Box 1.

The major source of nitrogen compounds in the environment is now the increasing use of fertilisers to enhance crop yields, driven largely by the need to feed the large proportion of the world's human population that is undernourished. This is a trend that is expected to continue (Figure 10).

(Source: Bumb and Baanante, 1996, Table 1) ©

Bumb, B. and Baanante, C. (1996) ‘Trends in Fertilizer Use and Projections to 2020’, Brief no 38, International Food Policy Research Institute

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Figure 10: Estimated growth in fertiliser use, 1959–2020

Other sources of nitrogen being added to the environment are urine and manure from livestock, industrial emissions and human waste. As a result of all this release of nitrogen compounds, the natural cycle of nitrogen in the environment has become swamped by what is called the ‘nitrogen cascade’ (Galloway et al., 2003). In the natural nitrogen cycle (Figure 11), de-nitrifying bacteria convert nitrogen compounds back into atmospheric nitrogen, but these are now unable to cope with the massive quantities currently being released into the environment. As a result, levels of nitrogen compounds are building up in soil, in the atmosphere and in water.

Figure 11: The natural (pre-industrial) nitrogen cycle

Effects on humans

The most well-known effect of nitrates on human health is methaemoglobinaemia (met-heem-oh-gloh-bin-eem-ia) or ‘blue baby syndrome’. If large quantities of nitrogen compounds are ingested in drinking water, the ability of the blood to carry oxygen is impaired, causing headache, fatigue, breathing difficulties, diarrhoea and vomiting and, in extreme cases, loss of consciousness and death. This syndrome is quite common in parts of the USA, and the Netherlands, where nitrogen levels are very high. Safety levels for nitrogen compounds in drinking water are set with a view to preventing methaemoglobinaemia in infants, an acute condition, and do not take into account the possible chronic health effects, for adults as well as children, of ingesting nitrogen compounds at low levels over a long period.

Nitrogen compounds are thought to be linked to asthma in some localities. Fresno, in the San Joaquin Valley of California, is notorious for its smog and has the third highest asthma rate in the USA. Fresno is the centre of a major milk-producing region and dairy farms release large amounts of nitrogen in the form of ammonia which forms particulates in the air (Hooper, 2006). The increased use of nitrogen compounds in agriculture is also indirectly implicated in the marked increase in the incidence of asthma in many developed countries. Crops grown in nitrogen-enriched soils grow profusely and their flowers release very large amounts of pollen into the air (Townsend et al., 2003).

The most serious health consequences for humans from nitrogen pollution may well arise indirectly through its effects on the environment. For example, high levels of nitrogen in water cause the formation of algal bloom (Figure 12), exceptional growths of plant-like organisms, called algae, often combined with bacteria, which ‘choke’ lakes, rivers and streams. These blooms may contain a type of bacteria, called cyanobacteria, which produce toxins, killing water life and posing a threat to people. Algal blooms can kill the fish stocks on which many people may be dependent for food and it can take many years to clear them and restore the natural water ecology.

(Photo: Michaek Marten/Science Photo Library) ©

Michael Marten/Science Photo Library

Figure 12: An algal bloom in a woodland pond. The pond has been turned green by the growth of algae, which covers the surface and prevents light and oxygen reaching plants and animals underneath