Addition of silver and copper ions to water
The addition of silver and copper ions to water (Figs 1 and 2) is a systemic method for long-term control of Legionella
(1, 5). In its guidelines, the World Health Organization (WHO) specifies maximum levels for silver (0.1 mg/l), and for copper (2.0 mg/l) in water (18). Copper levels higher than 1 mg/l may cause discolouration of laundry and sanitary equipment. At copper levels of over 2.5 mg/l, the water may acquire an undesirable bitter taste. At even higher concentrations, the water may become discoloured. There is a theoretical possibility of resistance to silver and copper developing, but so far this has not been found (5). A high water pH value may reduce the effect of copper ionisation (19).
Figure 1 Positively charged ions such as silver (Ag+) and copper (Cu2+) are attracted to the negatively charged cell walls of organisms. This electrostatic connection creates stress that causes the cell wall to break down. This, coupled with protein denaturation, causes cell lysis and prevents cell division.
Figure 2 Passing direct current through silver and copper electrodes that are immersed in water causes the release of positively charged ions into the water. When a stream of water is passed between the electrodes, the metallic ions will move into the water system before they reach the opposite electrode.
The anti-bacterial effect of silver and copper ions on Legionella and Mycobacterium was studied in 2001 in Finland
(20). Free Legionella in the water was found to be totally destroyed. For ionisation to be effective, it is important that water taps and showers be used regularly. Copper penetrates biofilm better than silver. The authors conclude that a well monitored ionisation system may be the solution to preventing nosocomial Legionnaires’ disease (20).
Any new method for combating Legionella should be subjected to a step-by-step standardised evaluation before it is introduced into hospitals. The method must have an in vitro effect on Legionella, and its efficacy must be demonstrated in controlled studies
(21). In a multi-centre trial on the efficacy of using silver and copper ions, no cases of Legionnaires’ disease were reported during the study period for over five years apart from a single case at a hospital immediately after installation of the system (21). All the hospitals had cases of Legionnaires’ disease before they installed the system. All had tried standard methods of controlling Legionella, such as shock chlorination and shock heat treatment, without success. The hospitals installed silver and copper ionisation systems in the period 1989 – 1995. Prior to the installation, 47 % of the hospitals reported Legionella contamination in more than 30 % of water samples. Legionella pneumophila serogroup 1 was reported in 75 % of cases. At the conclusion of the study in 2000, the hospitals had been using the method for seven years on average. At the start of the study in 1995, half of the hospitals reported that they had not experienced Legionella contamination of water after installation of the silver and copper ionisation system. When checked five years after the start of the study, 43 % of the hospitals had still had no Legionella contamination of their water. The remainder had Legionella occurrences in less than 30 % of their water samples. There were reports of water discolouration at high silver concentrations (21).
The addition of silver and copper to water is not generally permitted in Norway because of the effect it would have on waste water treatment and the possibility of depositing sewage sludge. However, the Norwegian National Institute of Public Health (FHI) and the Norwegian Food Safety Authority (NFSA) have concluded that the addition of limited amounts of silver and copper ions to water should be permitted to prevent susceptible groups of patients from contracting Legionnaires’ disease. Telemark Central Hospital applied for and was granted dispensation by the NFSA in 2008. Since 23 June 2009, the NFSA approves the use of copper and silver ions in internal water distribution facilities when the public health service believes that the quantity of Legionella in the water is so high that the health hazard can only be handled by means of such methods
Silver is absorbed via the gastrointestinal tract, mucous membranes and skin lesions. Most of the absorbed silver is excreted with gall in the faeces
(1, 18). FHI assessed that the planned silver concentration of 40 mg/l does not constitute a health risk for hospital staff and patients (1, 18).
Copper is an important trace metal, but also contaminates drinking water. Copper is primarily absorbed in the small intestine, and most of it is transported to the liver, where it is incorporated in proteins. Copper is excreted in faeces. There are documented cases of acute and chronic copper poisoning. For adults, long-term intake in concentrations of 1 – 10 mg/day is not harmful
(18). The EU has specified a maximum concentration of 2 mg/l copper in drinking water, which is consistent with the WHO’s drinking water guidelines of 2002 (18).
Silver and copper are heavy metals, and it is therefore desirable to reduce the quantity of these metals in nature. Silver accumulates in the soil to a greater extent than copper. Extensive use of silver, which goes into waste water, may contribute to environmental bacteria developing resistance to antibiotics
(5). This resistance mechanism can theoretically be transmitted to pathogenic bacteria.