Copper prevents growth of bacteria in HVAC systems, report concludes

The spread of infection through hospital heating, ventilation and air conditioning systems has long been a concern for estates and facilities managers, but research suggests the use of antimicrobial copper could limit the growth of harmful bacteria.

Dr Michael Schmidt, professor and vice chairman of microbiology and immunology at the Medical University of South Carolina, and his team have published a new study evaluating the efficacy of bug-busting copper in reducing contamination in HVAC systems.

They conducted a comparative study where heat exchangers fabricated from either antimicrobial copper or aluminium were evaluated for their ability to limit microbial growth, using a full-scale HVAC system under conditions of normal flow rates using single-pass outside air.

The conditions in HVAC systems would appear to be ideal environments for the growth and propagation of microbes

They found that commonly-used aluminium components developed stable biofilms of bacteria and fungi within four weeks of operation, whereas the antimicrobial properties of metallic copper were able to limit the bacterial load associated with the copper heat exchanger fins by 99.99% and the fungal load by 99.74% during the same time period.

The results, described by the researchers as ‘significant’, indicate a role for antimicrobial copper in reducing contamination on heat exchanger surfaces, which could potentially improve resultant air quality and increase efficiency in HVAC systems.

Dr Schmidt’s research comes after previous findings revealed a role for copper surfaces in fighting bacteria levels on surfaces in hospital intensive care environments. That comparison showed a 40% reduction in the risk of patients contracting a healthcare associated infection (HCAI).

The new latest paper states: “In densely-occupied buildings, airborne microbial contaminants can result in numerous adverse effects on human health and wellbeing, including inflammation and infections.

Microbial growth in heating ventilation and air-conditioning systems and subsequent contamination of the indoor air environment is of increasing concern

“Airborne bacteria and fungi have the potential to adversely impact human health by causing infections, allergic responses or toxic effects. Thus microbial growth in heating ventilation and air-conditioning systems and subsequent contamination of the indoor air environment is of increasing concern.

“The conditions in HVAC systems would appear to be ideal environments for the growth and propagation of microbes. Intrinsic microbial biofilms on air-handling exchanger coils are associated with lowered heat transfer efficiencies and increased corrosion, as well as potential odour issues. Thus, little or no growth of microorganisms on HVAC surfaces is optimally desired.”

Contaminants accumulate on heat exchanger coils and fins, in condensate drain pans, on air filters, and in air ducts. Indoor surfaces and building occupants can then be exposed to bio-aerosols from these sites.

Measurement of airborne fungi showed differences in concentrations and species diversity between adjacent regions of HVAC systems. Reduction of both total colony forming unit (CFU) counts and species diversity were greatest across the outdoor air intake. Further reduction was observed across the filters. Cooling coils also had substantial ‘filtration’ effects, demonstrating that significant fractions of viable airborne fungi are deposited within building HVAC systems.”

To test the impact of copper surfaces, chilled water was circulated through the experimental heat exchangers. After a week the system was temporarily stopped and 99 coupons of either copper or aluminium were installed into four equivalent copper and aluminium heat exchangers. Triplicate sets of coupons were then removed from each exchanger four weeks after their installation.

Construction of heat exchangers from copper profoundly inhibited the concentration and diversity of microbes associated with the biofilms formed

The results showed a robust biofilm was established on the aluminium heat exchanger, while the viable fungi and bacteria biofilms on the copper exchangers were at least one order of magnitude lower.

The report concludes: “It is acknowledged that while the complexities of the microbial communities resident on both types of heat exchangers warrant further study, the fundamental observation of this study was significant.

“Construction of heat exchangers from copper profoundly inhibited the concentration and diversity of microbes associated with the biofilms formed. The extent with which copper limited the growth of bacteria was 99.99 % and the limitation of fungal growth was found to be 99.74 % of that observed on the control aluminium-based heat exchangers.

The data supports the view of others that, when copper is substituted for aluminium in the construction of the heat exchangers, a substantial and significant reduction in the biofilm associated with the heat transfer device found in HVAC systems can be achieved.”

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