The Effectiveness of Air Treatment Technologies in Preventing Respiratory Infections

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In the wake of the Covid-19 pandemic, the deployment of Air Treatment Technologies (ATT) gained prominence as a practical measure to mitigate the spread of respiratory infections, particularly in environments where social distancing was challenging.

Technologies such as High Efficiency Particulate Air (HEPA) filtration and Germicidal Ultraviolet Light (GUVL) were proposed to either remove microbes from the air or render them incapable of causing infection. This article aims to provide a detailed examination of the effectiveness of ATT in real-world settings, considering various technologies, study designs, and stages of development.

HEPA Filtration:

One of the widely acknowledged air treatment technologies is HEPA filtration, designed to remove at least 99.97% of aerosols measuring 0.3 μm in diameter (US Department of Energy, 2005). Governments, including those of the United States (Camfil, 2021), Germany (Ulmair, 2021), and the United Kingdom (Zimmer, 2021), considered deploying HEPA systems, especially in schools where maintaining social distancing was challenging. However, concerns were raised about the high costs and uncertainties regarding the effectiveness of these devices (Brandon, 2020; Akpan and Jeffrey-Wilensky, 2021; Wightwick, 2021).

Germicidal Ultraviolet Light (GUVL):

An alternative approach to air treatment involves the use of Germicidal Ultraviolet Light (GUVL), which operates within safe bandwidths for chronic human exposure while deactivating viruses (Narita et al., 2020). The potential of GUVL in decontaminating air in the presence of people further fueled interest during the pandemic.

Challenges and Initiatives:

Despite the aspirations to deploy ATT, significant challenges emerged, including the high cost and uncertainty about the effectiveness of these technologies. To address these concerns, several cluster randomized controlled trials were initiated, examining the impact of HEPA or GUVL systems in schools (ISRCTN46750688; NCT05016271) and long-term residential care homes (ACTRN12621000567820; NCT05084898; ISRCTN63437172). However, as of now, the results of these trials remain pending.

Development Stages and Safety:

Introducing any novel technology or treatment requires rigorous development stages, including safety testing and real-world experiments. Technologies aimed at purifying or treating air are rapidly evolving and are concurrently in various stages of development. This underscores the importance of thorough testing before justifying large-scale population treatment.

Systematic Review:

To contribute to the understanding of ATT effectiveness, a systematic review was undertaken, considering evidence published from 1970 to late 2022. The review focuses on real-world settings, examining respiratory and gastrointestinal infection outcomes in humans following exposure to a broad range of potential technologies. Both observational study designs (cohort or case control) and experimental trials are considered, encompassing portable devices and permanent installations.

Discussion

In this discussion, we delve into the findings of our comprehensive review of Air Treatment Technologies (ATT) and their effectiveness in preventing respiratory infections, considering various technologies, study designs, and outcomes. We also address the challenges, gaps, and considerations for future research in this critical area.

Previous Reviews and Updated Inclusion Criteria:

Our literature search builds upon previous systematic reviews, such as Hammond et al. (2021), which highlighted the lack of studies investigating the incidence of respiratory infections using portable HEPA filter devices. Unlike prior reviews, our updated search includes a broader range of ATT, both portable and installed, and considers a more extensive array of outcomes, including respiratory symptoms and incidence of respiratory infections.

Effectiveness of ATT in Inactivating SARS-CoV-2:

Studies have widely reported the successful inactivation of SARS-CoV-2 in air samples and on surfaces through the use of ATT (Rodríguez et al., 2021; Myers et al., 2022; Zhang et al., 2022). These findings indicate the potential effectiveness of ATT in reducing microbe presence in the environment. However, our synthesis of symptom and infection outcomes does not conclusively support the notion that ATT significantly reduces respiratory or gastrointestinal infections.

Combined Technologies Show Promise:

Notably, our analysis suggests that the most significant reductions in symptoms and infections were observed in conjunction with combined technologies, such as ionizers with electrostatic cleaners or HEPA standard filters with additional charcoal-based filtration. This highlights the potential synergy of multiple ATT in enhancing their overall effectiveness.

Comparison with Livestock Farm Studies:

Studies on controlled swine farms demonstrated reduced clinical signs of enzootic pneumonia and other viral indicators among animals subjected to air filtration (HEPA or MERV rating 14/16). It is essential to acknowledge the key difference between livestock farm settings and human activities. While animals often reside in confined spaces for extended periods, humans, except for specific groups like prisoners and care home residents, typically have more varied indoor and outdoor activities.

Focus on Vulnerable Populations:

Our findings emphasize the potential success of ATT in settings where individuals spend a significant portion of their time indoors, particularly care homes. For instance, a study found that elderly care home residents spent an average of 95% of their time indoors, making them especially vulnerable to respiratory infections (Almeida-Silva et al., 2014).

Limitations of Existing Experimental Trials:

Despite the ongoing efforts in the form of registered cluster randomized controlled trials (RCTs) evaluating HEPA and GUVL technologies, the lack of rigorous experimental trials remains a challenge. Biases introduced in cohort studies underscore the importance of well-designed RCTs in assessing the true preventive effects of ATT. The trials underway in various settings should provide valuable insights into the diverse infrastructures and infection control policies.

Consideration of Allergic Response and Asthma Studies:

Our review incorporates studies conducted in the context of allergenic response or asthma unless explicitly excluding infection as a cause of symptoms. This inclusive approach ensures a comprehensive examination of respiratory and gastrointestinal outcomes in relevant settings, deviating from the original protocol.

Cost Considerations and Resource Limits:

The financial aspect of ATT implementation cannot be overlooked. As highlighted by previous reports (Wightwick, 2021; Zimmer, 2021), ATT can be expensive. Evaluations should include data on implementation costs, operational costs, and energy efficiency to inform decision-makers about the feasibility of widespread adoption.

Future Research Directions:

Future research should address the lack of studies on aerosols and gastrointestinal infections. Considering multiple pathogens, including those associated with projectile vomiting and norovirus, will contribute to a more holistic understanding of ATT benefits. Additionally, potential adverse effects, such as noise nuisance, should be systematically addressed, considering technological advancements that might mitigate these concerns.

In conclusion, while ATT, including HEPA filtration and GUVL, show promise in reducing microbe presence, their definitive effectiveness in preventing respiratory and gastrointestinal infections requires further investigation. Ongoing and future experimental trials, coupled with a focus on cost considerations and potential adverse effects, will contribute to a more nuanced understanding of the role of ATT in public health interventions.


reference link : https://www.sciencedirect.com/science/article/pii/S0091743523003602#s0075

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