Both copper and silver antimicrobial efficacy can be affected by temperature and humidity. In general, higher temperatures can accelerate ion release from both copper and silver surfaces, thus enhancing their antimicrobial activity. Humidity can also play a role, as moisture on the surface can facilitate ion release and enhance antimicrobial efficacy.
But what happens at room environmental conditions? “Room environmental conditions” typically refer to the ambient conditions within an indoor space such as a room or building, and it is under these ambient conditions that you need to achieve the potent antimicrobial effect of the silver or copper containing products. The temperature of the room, measured in degrees Celsius (°C) or Fahrenheit (°F), can vary depending on factors such as climate control systems, outdoor temperature, and building insulation.
In hospitals in the USA, indoor temperatures and humidity levels are typically regulated to ensure patient comfort, staff productivity, and the optimal functioning of medical equipment. While exact ranges may vary depending on specific hospital policies, local climate conditions, and the needs of patients, the recommended temperature range in hospital environments is typically between 20°C (68°F) and 24°C (75°F). This range is chosen to provide a comfortable environment for patients, visitors, and staff while also minimizing the risk of infections and ensuring the proper functioning of medical equipment. And the recommended relative humidity (RH) level in hospitals usually falls between 30% and 60%. This range helps maintain a comfortable indoor environment while also minimizing the growth of mold, bacteria, and other pathogens. Additionally, proper humidity levels can help prevent discomfort associated with dry air, such as irritated respiratory passages and skin.
Thus, the question is – are there any differential antimicrobial efficacies between copper and silver containing products at typical hospital indoor room temperatures and humidity conditions?
In order to address this question, Michels HT and colleagues tested the antimicrobial efficacy of two silver-containing products, one incorporating ‘silver ions in a zeolite carrier’ and the second ‘silver ions as the active ingredient, in an organic matrix’ in comparison to 5 different copper alloys containing between 65% to 99.9 copper, under typical indoor environmental conditions (Michels HT, Noyce JO, & Keevil CW (2009) Effects of temperature and humidity on the efficacy of methicillin-resistant Staphylococcus aureus challenged antimicrobial materials containing silver and copper. Lett Appl Microbiol 49(2):191-5. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2779462/).
Triplicate coupon sample from each material were inoculated with very high titres (2×107 Colony Forming Units (CFU)/coupon) of methicillin resistant Staphylococcus aureus (MRSA). The inoculated samples were exposed to different temperatures and RH conditions for varying times ranging from 15 minutes to 6 hours. The bacteria were then recovered and their viability determined.
While the copper alloys showed a 7-log reduction (99.99999%) of the MRSA viable titres in 75 minutes at 22°C and 50% RH, no meaningful reductions were seen at 360 minutes in the silver coupons. Even at longer incubation times (24 hours) both silver products showed less than 0.3 log reduction at 90% RH at 20°C and 35°C, and no reduction at all at 22% RH and 35°C. In contrast, the copper allows demonstrated > 5 log reduction under all test conditions.
The high efficacy levels displayed by the copper alloys, at temperature and humidity levels typical of indoor environments, compared to the low efficacy of the silver ion-containing material under the same conditions, favours the use of copper alloys as antimicrobial materials in indoor environments such as hospitals. Indeed, copper alloys and polymeric surfaces containing copper oxide microparticles have received EPA approval to make public health claims (EPA registrations 85012, 1-6 and 84542-7, respectively) and thus in many hospitals metallic copper and polymeric surfaces containing copper oxide microparticles were installed in frequently touched surfaces [Coppin JD, et al. (2017) Self-sanitizing copper impregnated surfaces for bioburden reduction in patient rooms. Am J Infect Control 45(6): 692-694.]. In addition, many hospitals surround the patients with antimicrobial copper oxide containing medical textiles (Borkow G (2014) Biocidal hard and soft surfaces containing copper oxide particles for the reduction of healthcare-acquired pathogens. Use of Biocidal Surfaces for Reduction of Healthcare Acquired Infections 1(5): 85-101.https://link.springer.com/chapter/10.1007/978-3-319-08057-4_5).
In summary, copper containing materials are more appropriate for applications in room environmental conditions and accordingly, several studies have shown the efficacy of copper containing materials in reducing bioburden in hospital environments and reducing hospital acquired infections (e.g. Lazary A, et al. (2014) Reduction of healthcare-associated infections in a long-term care brain injury ward by replacing regular linens with biocidal copper oxide impregnated linens. Int J Infect Dis 24: 23-29; Harold TM et al. (2015) From Laboratory Research to a Clinical Trial: Copper Alloy Surfaces Kill Bacteria and Reduce Hospital-Acquired Infections. HERD. 9(1): 64–79; Marcus et al. (2017) Reduction of healthcare-associated infection indicators by copper oxide impregnated textiles: crossover, double-blind controlled study in chronic ventilator-dependent patients. American Journal of Infection Control 45(4): 401–403; Arendsen LP et al., (2019) The Use of Copper as an Antimicrobial Agent in Health Care, Including Obstetrics and Gynecology. Clin Microbiol Rev. 32(4): e00125-18).
