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.