Energy Recovery Ventilation: What Is Needed to Fill the Research Gaps Related to Its Effects on Exposure to Indoor Bio-Aerosols, Nanoparticulate, and Gaseous Indoor Air Pollution

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The author presented an important point to be aware of regarding the increased usage of heat exchange devices. The contents and intentions are precise; however, a few minor points need to be explained or given more information as potential risk factors.

1) The figures for three ERV types—plate and energy wheel exchangers, membrane exchangers, and fixed-bed regenerators—would be helpful. If space is limited, supplementary documents can be utilized.

2) As the composition of air components, bioaerosol requires greater attention due to its potential to multiply or grow to increase the number and/or amount of biological substances under certain conditions. It is important to highlight the nature of microorganisms; if the conditions allow, they can multiply and generate more biological materials to cause more significant health effects.

 

3) Linked with the above point 2), the heat exchange area may experience water condensation on the warmer side. If the condensation spot or high humidity area occurred, it could be a spot for the microorganism to grow further. Please indicate if any preventative measures are considered to reduce the potential source of toxins that can cause health effects. 

Author Response

Reviewer 1

The author presented an important point to be aware of regarding the increased usage of heat exchange devices. The contents and intentions are precise; however, a few minor points need to be explained or given more information as potential risk factors.

Response: We thank the reviewer for the constructive comments. We considered the reviewer’s suggestions and have made corresponding modifications and amendments to the manuscript, including, specifically, discussing in more detail the different types of ERVs and the specific factors related to the operation of each of them, discussing the different types of air pollutants and their chemical identity affected by ERV operation in more detail, the specific phenomena responsible for the cross-contamination of the incoming fresh air flow with pollutants from the outgoing stale air, and the problem of liquid water condensate and frost formation inside ERVs and its potential to lead to indoor air contamination.

• The figures for three ERV types—plate and energy wheel exchangers, membrane exchangers, and fixed-bed regenerators—would be helpful. If space is limited, supplementary documents can be utilized.

Response: We thank the reviewer for this suggestion. As the different types of ERVs are clearly described with corresponding descriptive images in the handbook “Air-to-Air Energy Recovery Equipment” from the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE), we have included a reference to this handbook to guide the reader to the relevant details about the different types of ERVs.

• As the composition of air components, bioaerosol requires greater attention due to its potential to multiply or grow to increase the number and/or amount of biological substances under certain conditions. It is important to highlight the nature of microorganisms; if the conditions allow, they can multiply and generate more biological materials to cause more significant health effects.

Response: We thank the reviewer for this suggestion. We have included the following information that addresses the effect of condensation on potential biological growth in ERVs. We note that the lack of knowledge regarding the phenomenon of biological growth in ERVs, including the types of organisms, conditions leading to different biological growth and its effect on indoor air pollution is one of the research gaps that must be addressed, which we highlight in this perspective.

“One of the effects of recovering moisture using ERVs is the condensation and frosting of this moisture in the heat exchanger during the winter in northern climates under mild and extremely low temperature conditions, respectively [22]. Condensation of moisture can lead to corrosion and microbial growth leading to detrimental health effects due to these biological indoor contaminants [45]. Preheating of the outdoor air has been suggested as a potential solution to prevent condensation [45] and frosting [46], and the associated microbial growth.”

 3) Linked with the above point 2), the heat exchange area may experience water condensation on the warmer side. If the condensation spot or high humidity area occurred, it could be a spot for the microorganism to grow further. Please indicate if any preventative measures are considered to reduce the potential source of toxins that can cause health effects. 

Response: The following information has been included in the section described above that describes the measures currently used to tackle the problem of condensation that may lead to microbial growth.

“Preheating of the outdoor air has been suggested as a potential solution to prevent condensation [45] and frosting [46], and the associated microbial growth.”

Reviewer 2 Report

Comments and Suggestions for Authors

General Comments

The manuscript, titled "Energy Recovery Ventilation: What is Needed to Fill the Research Gaps Related to its Effects on Exposure to Indoor Bio-aerosols, Nanoparticulate, and Gaseous Indoor Air Pollution", addresses an important topic concerning energy recovery ventilation (ERV) systems and their impact on indoor air quality (IAQ). The authors have identified significant knowledge gaps related to ERV performance, including pollutant crossover and transformation within ERVs. The manuscript provides a well-structured discussion on the need for research into ERV-related pollutant dynamics and suggests advanced analytical methods for addressing these gaps. The study is conceptually relevant and timely. However, several areas require improvement in terms of logical coherence and clarity of presentation. Overall, a minor revision is recommended to address the outlined concerns.

Major Concerns

The introduction reiterates general knowledge about IAQ and ERVs, which could be condensed to make room for a deeper discussion on nano aerosols and VOCs.

The manuscript suggests advanced techniques like GC-MS and TEM-EDS but does not specify how these techniques will be implemented to address the identified gaps.

The manuscript lacks actionable insights into how the findings will inform ERV design and operation.

The manuscript refers to global IAQ challenges but does not specify how findings would vary across different climates or building types.

Focus more on how findings will translate into actionable recommendations for ERV deployment.

 

Author Response

Reviewer 2

General Comments

The manuscript, titled "Energy Recovery Ventilation: What is Needed to Fill the Research Gaps Related to its Effects on Exposure to Indoor Bio-aerosols, Nanoparticulate, and Gaseous Indoor Air Pollution", addresses an important topic concerning energy recovery ventilation (ERV) systems and their impact on indoor air quality (IAQ). The authors have identified significant knowledge gaps related to ERV performance, including pollutant crossover and transformation within ERVs. The manuscript provides a well-structured discussion on the need for research into ERV-related pollutant dynamics and suggests advanced analytical methods for addressing these gaps. The study is conceptually relevant and timely. However, several areas require improvement in terms of logical coherence and clarity of presentation. Overall, a minor revision is recommended to address the outlined concerns.

Response: We thank the reviewer for the insightful comments and suggestions. We have carefully considered the reviewer’s suggestions and have made corresponding modifications to the manuscript.

Major Concerns

The introduction reiterates general knowledge about IAQ and ERVs, which could be condensed to make room for a deeper discussion on nano aerosols and VOCs.

Response: As suggested by the reviewer, we have updated the introduction to expand the discussion on nano and bioaerosol contaminants and VOCs in indoor air. We thank the reviewer for this suggestion. The updated text is copied below for the reviewer’s reference.

“Air pollutants can infiltrate indoor spaces from the outside [9]. In addition, indoor air pollution sources and poor ventilation also cause elevated concentrations of air pollutants indoors: from carbon dioxide to volatile organic compounds (VOCs) and aerosol particles [10]. In addition to the well-known harmful air pollutants, increasing evidence links the negative impact of two specific types of emerging indoor air contaminants on human health and well-being: a wider range of chemical identities of VOCs and nanoaerosols (ultrafine particles) [11]. Indoor concentrations of VOCs, released from household products like disinfectants, aerosol sprays, paints, and dry cleaned clothing, have been shown to be up to 10 times greater than outdoors [12]. Chronic exposure to VOCs has been associated with cancer [13] and damage to the central nervous system [14]. VOCs and high carbon dioxide concentrations indoors reduce productivity [15] and diminish cognitive function and psychomotor performance [16-18]. Comparison of number-based concentrations of nanoaerosol particles showed that indoor concentrations were higher than outdoors, with higher concentrations likely arising from indoor sources of these pollutants (Table 1) [19].  High levels of indoor nanocluster aerosols derived from propane combustion during cooking have been observed resulting in higher exposure of adults and children to these pollutants.”

The manuscript suggests advanced techniques like GC-MS and TEM-EDS but does not specify how these techniques will be implemented to address the identified gaps.

Response: We have updated the section describing the analytical techniques to clarify the usefulness of each technique and how it can be used in the design and development of novel materials for ERVs based on the effectiveness of these materials in removing particulate and other pollutants. The updated section is copied below for the reviewer’s reference.

“Advanced analytical techniques are needed for research that can address the above-mentioned knowledge gaps, for which measuring and characterizing particulate and gaseous toxic and carcinogenic organic compounds with a wide range of chemical identities is required. Historically, advanced analytical techniques have seen minimal use in the HVAC field. However, many such methods that can be used for determining real-time pollutant levels and their chemical identities have become more economical and accessible. Specifically, fast mobility particle sizing (FMPS), scanning mobility particle sizing (SMPS) techniques, and diffusion-charger-based aerosol sizers can be used to measure real-time aerosol size distributions of emissions from ERVs. Electrochemical, catalytic, infrared real-time sensors and gas chromatography-mass spectrometry (GC-MS) can be used to determine the chemical identity and chemical composition of aerosol pollutants. Transmission electron microscopy (TEM) coupled with electron dispersive spectroscopy (EDS) can provide high-resolution imaging of nanoaerosols and elemental analysis of individual particles with EDS. These analytical tools can be used to test ERVs during the development process to gain critical insights into pollutant releases from different types of ERVs themselves, the chemical identities of deposits on surfaces inside ERVs, and the physical and chemical transformations of these pollutants. Results from these analyses can be used to develop materials or production strategies to minimize pollutant crossover into the air inflow. Integrating these diverse analytical techniques with novel experimental approaches will also aid future industrial and academic research in the broader HVAC field.”

The manuscript refers to global IAQ challenges but does not specify how findings would vary across different climates or building types.

Response: We have updated the manuscript to highlight the following:

1) clarify the problem with IAQ in different climates such as cold climate

“In seasonally cold and seasonally hot climates, people keep windows and doors closed most of the year to conserve the expensive energy they use to heat or cool their homes. Extensive research has shown that breathing stagnant air leads to a series of acute and chronic adverse effects. These negative effects include difficulty breathing, poor quality of sleep, nasal congestion, headaches, throat and eye irritation, lower productivity, reduced cognitive ability, including poorer learning outcomes in children, and sick and tight-building syndromes.”

2) highlight the problem with current ERV design leading to frosting/condensation that can promote microbial growth, and 3) current solutions to address these problems.

“One of the effects of recovering moisture using ERVs is the condensation and frosting of this moisture in the heat exchanger during the winter in northern climates under mild and extremely low temperature conditions, respectively [27]. Condensation of moisture can lead to corrosion and microbial growth leading to detrimental health effects due to these biological indoor contaminants [49]. Preheating of the outdoor air has been suggested as a potential solution to prevent condensation [49] and frosting [50], and the associated microbial growth.”

The manuscript lacks actionable insights into how the findings will inform ERV design and operation. Focus more on how findings will translate into actionable recommendations for ERV deployment.

Response: We have updated the manuscript to include the following actionable recommendations for the design and deployment of ERVs. We thank the reviewer for this important suggestion.

“The outcomes of this research will inform the industry to optimize ERV design and operation to minimize adverse health effects associated with indoor air pollution. Addressing the research gaps will facilitate the design and development of novel materials that maximize moisture recovery and thermal recovery while minimizing retention and concentration of aerosol and VOC pollutants. This knowledge will enable the development of next-generation ERVs that incorporate membrane materials and surface coatings with antimicrobial properties and mitigate and capture and/or degradation of VOCs. These efforts will actively enhance public and occupational health while meeting energy efficiency goals.”

Reviewer 3 Report

Comments and Suggestions for Authors

This article highlights the need for research into the hazards that arise when air is introduced into residential buildings using mechanical ventilation with heat recovery. It presents a review of the literature and a brief description of the measurement methods used in the analysis of air pollution. Table 1 presents the results presented in the previous article, which were not discussed. I am not convinced that this article provides significant scientific information. The conclusions present only a rough outline of the research that should be carried out. I do not evaluate the article positively.

Author Response

Reviewer 3

This article highlights the need for research into the hazards that arise when air is introduced into residential buildings using mechanical ventilation with heat recovery. It presents a review of the literature and a brief description of the measurement methods used in the analysis of air pollution. Table 1 presents the results presented in the previous article, which were not discussed. I am not convinced that this article provides significant scientific information. The conclusions present only a rough outline of the research that should be carried out. I do not evaluate the article positively.

Response: We thank the reviewer for their opinion. We would like to clarify that this is a perspective and not an original research article. We have strengthened the manuscript based on the constructive comments provided by all reviewers and trust that this addresses the reviewer’s issues. With regards to the table presented, Table 1 with selected data is adapted from another publication, with the appropriate attribution provided for illustration of one of the problems discussed in this perspective. We identified key challenges with the existing state of knowledge of energy recovery ventilation technologies and are proposing a novel research direction and suggesting methodological approaches that could be used to tackle the identified research gaps. Our goal with this perspective is to provide highlight the current applications of ERVs in improving indoor air quality, and to highlight the need to design ERVs that address persistent issues with the current design and implementation of ERVs.

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The authors amended the reviewer's comments appropriately.

No further comments.