Using copper in heating and air-conditioning products can fight bacteria and improve air quality
Many microbes, particularly certain species of fungi and bacteria, are known to thrive in dark, moist environments - like those in HVAC systems.
Studies have found high concentrations of pathogens on various HVAC components, including heat-exchanger fins, condensate drip pans, drains and cooling coils. Once a colony of pathogens is established, organisms can spread throughout a building via the HVAC system.
While microbes in HVAC systems are often benign, some are linked to serious health problems. They can contribute to sick-building syndrome and building-related illnesses that can result in loss of work, wages, productivity, even hospitalization or death.
HVAC design and operation is an important element in maintaining healthy indoor air quality, partly because these microorganisms can become airborne and pass through ductwork to public spaces. Concentrations of microbes have been found in air samples of homes and commercial buildings.
The scientific community has long recognized copper's inhibiting effect on fungi and bacteria. This knowledge has recently prompted scientists at the University of Southampton in the United Kingdom to conduct research on the anti-microbial properties of copper and copper-alloy surfaces in ambient and chilled-air environments. This article will discuss various methods currently used to protect HVAC systems from harmful pathogens, the types of microorganisms commonly found in HVAC systems; their low survival rates on copper and copper alloy surfaces; and how this may impact indoor air quality.
DefensesThe first line of defense against harmful pathogens is a clean, well-maintained HVAC system with good air distribution. Current methods of protection include particulate control devices, chemical cleaning agents and treated surfaces. Each of these approaches, however, has its limitations.
Particulate control devices include filters, air cleaners and ultraviolet lights. The American Society of Heating, Refrigerating and Air-Conditioning Engineers has suggested that air filters are partly to blame for poor indoor air quality because of the microbiological particulates that stay on the surface of the filters. Ultraviolet germicidal irradiation systems can be an effective aid to good filtration, but, due to space limitations and cost, many systems are not properly configured.
Also, UV lights may be ineffective against hardier microbes. According to the Air-Conditioning and Refrigeration Technology Institute, "Vegetative bacteria are most susceptible to UVGI, while bacteria and fungal spores are substantially more resistant."
Cleaning, disinfecting and sanitizing chemicals have been promoted as anti-microbials for HVAC uses. Some of these products may be useful in reducing bacteria or fungi that can compromise the efficiency of the HVAC system. However, the U.S. Environmental Protection Agency has not studied the effectiveness of any anti-microbial products for use in HVAC systems for protecting public health, such as improving indoor air quality. Of additional concern is that the EPA has not assessed the potential exposure and associated risks to building occupants related to the use of these chemicals. In fact, the EPA has partnered with the National Air Duct Cleaners Association to educate HVAC-cleaning professionals about the proper use of anti-microbial agents.
LimitationsAnti-microbial products currently being used in HVAC systems may claim, at best, that they limit the growth of microbes that may harm the system itself. The effectiveness of these treated surfaces in the fight against the growth of harmful pathogens is uncertain and, as of yet, unapproved by government scientists.
Specific pathogenic risks in HVAC systems include legionellae, mycobacterium, and molds such as aspergillus niger. It has been shown that legionella pneumophila, the bacteria that causes Legionnaires' disease, can survive and grow at temperatures as high as 122°F. L. pneumophila is able to grow in materials that are commonly used in cold or warm potable water supplies, including mild and stainless steel, polypropylene, polyethylene, unpolymerized polyvinyl chloride, chlorinated polyvinyl chloride, and the jointing compounds, latex and ethylene-propylene copolymer. Research has shown, however, that L. pneumophila cannot survive on copper surfaces.
Apsergillus niger is one of the most common and virulent molds found in HVAC systems. Dr. C.W. Keevil, of the School of Biological Sciences at the U.K.'s University of Southampton, placed A. niger spores on aluminum and copper coupons and kept them at a temperature of 68°F. At the end of six hours, there was no change in the spores on the aluminum, while on the copper they had been completely eradicated (see chart).
Mycobacteria also appear to be able to grow on a variety of plumbing materials. Although their survival rates on the surfaces of plumbing materials have not been studied as thoroughly as L. pneumophila, it is known that some mycobacteria are susceptible to dissolved copper salts. Indeed, research has been able to show that there were less mycobacteria on copper surfaces compared to plastic in water treatment plants, domestic water supply systems and aquariums.
StudiesBecause indoor pathogens can become airborne, a study was conducted on the deposit of biological aerosols in fin and tube heat exchangers commonly found in HVAC systems. Cooling coils and heat exchangers were found to be major deposition points for biological aerosols of a wide range of fungi and bacteria. The researchers concluded that aluminum fins on the copper tubes in heat exchangers - rather than the copper tubes - are sources of significant microbial populations that can cause indoor air quality problems.
The biological contamination of coils can take place even when indoor-air microbial populations are relatively low. This is because the volume of air passing over the coils is extremely high, thereby enabling bacteria to build up over time. Most cooling coils are inaccessible for regular cleaning. When they are cleaned as part of service and maintenance operations, potentially allergenic and harmful microbes can be re-released into the air. Current studies suggest that a preferable solution to this problem is to use copper components or to coat or plate HVAC components with an anti-microbial copper product.
A wide body of research, as well as abundant experience in the commercial HVAC market, indicates that copper fin and tube heat exchangers are unmatched in terms of thermal performance, durability, efficiency and life-cycle costs. These factors alone make a compelling case for the preference of copper fins and tubes in HVAC systems. But the hygienic properties of copper identified in these studies make the case for copper tubes and fins even stronger.
The dark, humid environments of fin and tube heat exchangers are favorable for microbial growth. Heat exchangers made with copper can control these microbial populations. Copper has been demonstrated to exhibit effective antimicrobial properties over the operating temperatures and humidity environments of HVAC fins and tube. Microbes on aluminum, on the other hand, can live for months at these temperatures.
Existing data on copper surfaces at room and chilled temperatures suggest that tube and fins made from copper are an effective means to suppress microorganisms. Therefore, the use of copper for these components is a conscientious decision that can be made to reduce the growth and buildup of bacteria in HVAC systems that may adversely affect indoor air quality.