Methaemoglobinaemia
Most countries have limits on nitrate levels in drinking water, to reduce the risk of methaemoglobinaemia (also called methemoglobinemia, “blue baby syndrome”) in infants. Early research suggested a link between nitrate levels and methaemoglob- inaemia in formula-fed infants (Comly, 1945).
Figure 2.6: N2O flux following the application of urine in the spring, with or
without DCD. Treatments are N, Urea + Urine; • , Urea + Urine + DCD (3 different DCD application regimes) (Di and Cameron, 2003)
However cases of methaemoglobinaemia have mainly been reported from pri- vate wells where there may be bacterial contamination as well (World Health Or- ganization, 2007), with bacteriological contamination of water now being strongly implicated in methaemoglobinaemia. Addiscott and Benjamin (2004) states that “More recent surveys have shown no correlation between methaemoglobinaemia and nitrate in water until the concentration of nitrate exceeds 100 mg L−1” (22.6 mg NO−3-N L−1), with concentrations this high being possibly associated with bac-
terial pollution. Formula-fed infants are particularly susceptible to gastrointestinal infections, which will further increase the risk of methaemoglobinaemia if bacterial contamination of water is a contributing factor (World Health Organization, 2007). Methaemoglobinaemia is a problem primarily for formula-fed infants below the age of three months, as nitrate levels in breast milk are low. Few cases of met- haemoglobinaemia have been reported in children over the age of one, and the only cases reported in adults have been from very high doses of nitrate from accidental poisoning or medical treatments (World Health Organization, 2007).
Nitrate has been reported to cause methaemoglobinaemia in ruminant animals grazing high nitrate pastures, without reported bacterial contamination (Garner, 1963). However ruminants are capable of consuming large amounts of nitrate on
a regular basis, and it is likely that this condition is caused by the “sudden intro- duction of a readily consumed high-nitrate feed” rather than through exceeding a set intake level of nitrate (Sinclair and Jones, 1964).
There also appears to be no link between nitrate and cancer (World Health Or- ganization, 2007), and some researchers suggest nitrate may actually have benefits for human health (Addiscott and Benjamin, 2004).
Accordingly, the World Health Organisation (WHO) recommends that form- ula-fed infants not be given water exceeding 100 mg NO−3 L−1 (22.6 mg NO−3-N L−1), with a guideline value of 50 mg NO−3 L−1 (11.3 mg NO−3-N L−1) above which parents and authorities must be particularly vigilant against gastrointestinal infections (World Health Organization, 2006, 2007). The WHO also recommends boiling of water for making infant formula where there is a risk of microbiological contamination, and if drinking water exceeds 100 mg NO−3 L−1 alternative water sources should be used for infant formula if these are available.
Because methaemoglobinaemia is a problem only with formula-fed babies, it is important to consider the societal influence on methaemoglobinaemia risk as well. Ireland has the lowest rates of breastfeeding in Europe, with only 44% of moth- ers exclusively breastfeeding when they leave hospital (ESRI, 2006). Of the remain- ing mothers a small number are offering both breast-milk and formula (<5%) and the remainder feeding only formula (ESRI, 2006). The majority of those mothers exclusively breastfeeding when leaving hospital appear to either introduce form- ula “top-ups” or give up breastfeeding entirely over the following 6 weeks or so, however accurate statistics are hard to obtain. By way of comparison, in New Zealand virtually all babies are exclusively breastfed on leaving hospital, with 66% still exclusively breastfed at 6 weeks, and 51% at 3 months (New Zealand Ministry of Health, 2002).
The World Health Organisation recommends babies are exclusively breastfed to the age of 6 months (World Health Organization, 2002) - a practice which if followed would virtually eliminate the risk of methaemoglobinaemia as well as having many other health benefits. But there is far less visible promotion of this recommendation in Ireland than in New Zealand (personal observation of both health systems), and instead formula is actively promoted on television and even sometimes in pamphlets in doctors waiting rooms.
As a result, the vast majority of Irish babies receive formula or water before the age of 3 months, placing them at risk of methaemoglobinaemia if their water sources are not free of bacteria and low in NO−3. Avoidance of methaemoglobinaemia is therefore a higher priority in Ireland than in most other countries.
Having said that, despite the high risk factors in Ireland I have yet to find a single reported case of drinking water nitrate induced methaemoglobinaemia in Ireland. Furthermore, alternative sources of water are readily available in Ireland, with large quantities of bottled water available cheaply in all supermarkets, so the risk of methaemoglobinaemia can be readily avoided among formula-fed infants even when the water drunk by the rest of the family contains a high level of nitrate. In order to be certain to avoid methaemoglobinaemia, most countries have set standards for drinking water that are below the WHO limit. For example, the USA uses 10 mg NO−3-N L−1 (44% of the WHO limit of 22.6 mg NO−
3-N
L−1), while China uses 20 mg NO−3-N L−1 (88% of the WHO limit) (Hu et al., 2005). Interestingly, New Zealand defines 11.3 mg NO−3-N L−1 (50% of the WHO limit) as the limit for short-term exposure only with no value defined for long-term consumption (New Zealand Ministry of Health, 2005). The New Zealand standards are unclear as to whether this value applies to people other than formula-fed infants. The EU has imposed a Maximum Allowable Concentration (MAC) for drinking water of 50 mg NO−3 L−1 (11.3 mg NO−3-N L−1) (The Council of the European Union, 1998), with a guideline value of half this (5.65 mg NO−3-N L−1) and nitrate leaching from farms must currently be managed in order to comply with the MAC (Addiscott and Benjamin, 2004). Note that the EU maximum limit is half the WHO limit, and the EU guideline value is half the WHO guideline value, with these values being applied to all people (not just infants), making the EU MAC a very stringent standard when compared to the WHO recommendations.
In summary, if any country has a high risk of methaemoglobinaemia it would be Ireland, due to the low breastfeeding rates in this country. However it is very easy to avoid methaemoglobinaemia in Ireland even if drinking water is contaminated - just boiling water for formula to remove bacterial contamination (a common practice) should greatly reduce the risk of methaemoglobinaemia unless nitrate levels are over twice the MAC, and exclusive breastfeeding would eliminate the risk. It is very difficult to justify the stringent EU guideline value of 5.65 mg NO−3-N L−1on
public health grounds.
Environmental problems
The undisputed issue with nitrate leaching is eutrophication of surface waters. High nitrate (and phosphate) levels in lakes and streams can increase the growth of aquatic plants and algae, altering the aquatic ecosystem. This has many negative effects on biodiversity, and also on fisheries, shipping and the recreational value of waterways. Eutrophication in itself is a serious enough reason to try to reduce nitrate losses from agricultural land into water bodies.
In order to determine what stocking rates or fertiliser levels are acceptable without causing eutrophication, it is necessary to know how much NO−3 can be lost without causing eutrophication.
Jin and Tu (1990), cited in Cheng and Li (2006), outline criteria to evaluate the trophic state of lakes in China. In their criteria a lake may be eutrophic if the total N exceeds 1.4 mg L−1.
However Cognetti (2001) points out that “...mere determination of nutrient and chlorophyll concentration in the waters is not sufficient to provide information on the severity of eutrophication. Indeed, in eutrophic ecosystems, a very low con- centration of nutrients is generally found, because they are stored in sediments...”. The concentration of N observed in a eutrophic ecosystem may have little relevance to the maximum allowable concentration in waters entering that ecosystem. If only a small proportion of the N is in solution, it may have taken far larger inputs of N than are indicated by the water content alone to have caused the system to become eutrophic. It may also take far lower inputs of N than this to correct the situation, because the total N content of the system is now so elevated.
The level of NO−3 input that can occur before causing eutrophication will vary greatly between ecosystems. It is probably counterproductive to attempt to come up with one threshold value, rather values need to be found for individual ecosys- tems that require protecting. This will be a very complicated task.
At present there are no legislative values in Europe for maximum nitrate con- centrations entering aquatic environments, the only legislative nitrate value being the MAC drinking water standard, which may have little relevance to aquatic ecosystems.