Non-commercial Air Purifier—The Effectiveness and Safety

Round 1

Reviewer 1 Report

The authors have put considerable effort to follow the reviewers' suggestions and now the manuscript has been greatly improved. So, my opinion is that it currently meets the publication standards of Buildings.

Author Response

May 25^th, 2020

Dear Reviewer 1

Thank you for your comments and helpful suggestions for improving the presentation of our study.

Yours sincerely,

Anna Mainka

Reviewer 2 Report

The paper should take in consideration, at least, in conclusions, about the present emergency (COVID) and considerations regarding virus and only bacteria.

The health implication is a good content, I suggest to add a scheme with the best practices for the human occupancy; it can be very smart and clear (concise).

The work is very interesting in general by from my point of view it needs to be more and more attractive with the future application from the laboratory to the real space.

In any case is a very well-done work, although the solution with O₃ is not a solution I prefer.

Author Response

May 25^th, 2020

Dear Reviewer 2

Thank you for your comments and helpful suggestions for improving the presentation of our study.

The paper should take in consideration, at least, in conclusions, about the present emergency (COVID) and considerations regarding virus and only bacteria.

We have added this fragment in conclusions:

Although our ozone generator is dedicated to killing bacteria and fungi indoors, a new global epidemic of coronavirus requires consideration of whether this device can also be used to destroy coronavirus. The new coronavirus is considered to be an "enveloped virus". Enveloped viruses are usually more sensitive to physicochemical challenges than naked viruses (without an envelope). In addition, ozone has been shown to kill the SARS coronavirus. Since the structure of the new 2019-nCoV coronavirus is almost identical, we can believe that it will also work on the new coronavirus. It is also known that ozone destroys this type of virus, breaking through the outer shell to the core, causing damage to the viral RNA. Ozone can also damage the outer layer of the virus in a process called oxidation. However, this problem requires further investigation.

The health implication is a good content, I suggest to add a scheme with the best practices for the human occupancy; it can be very smart and clear (concise).

We have added the scheme with the best practices for human occupancy.

The work is very interesting in general by from my point of view it needs to be more and more attractive with the future application from the laboratory to the real space.

We agree that the use of such solution in real environment is a promising direction of our future work.

In any case is a very well-done work, although the solution with O₃ is not a solution I prefer.

Again, we would like to express our appreciation for your efforts and helpful comments.

Please find the revised version of our paper enclosed.

Yours sincerely, Anna Mainka

Author Response File: Author Response.pdf

This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.

Round 1

Reviewer 1 Report

The authors have addressed most concerns adequately and significantly improved the manuscript. Reading flow is now easier, results presented in a more convincing way and conclusions are therefore supported adequately by the findings.

Author Response

Dear Reviewer 1

Thank you for your comments and helpful suggestions for improving the presentation of our study.

Yours sincerely,

Anna Mainka

Reviewer 2 Report

The authors proposed a homemade air purifier that primarily consisted of a duct fan and ozone generator with a mesh filter to remove coarse particulates. 

The purpose of the ozone generator is to generate sufficient amounts of ozone to kill airborne microorganisms/bioaerosols. 

The reviewer has several serious concerns regarding the nature of the work.

(1) Ozone is a well-known oxidizing and hence sterilizing agent. It is commonly used in the sterilization of water in swimming pools as well as in commercial passenger airliners air sterilization system. Therefore the reviewer's concern regarding this work is not the use of ozone but the manner in which the authors chose to use it. 

The design proposed by the authors is to simply eject quantities of ozone into an enclosed space. This is a very dangerous proposition no matter how well intended the authors are. This design is not very different from a gas chamber. 

(2) In order to prevent the authors' design from turning a room into a gas chamber, ozone sensors will have to deployed throughout room as well as adjacent rooms. Only with ozone sensors that is feedback to the ozone generator will ensure that the ozone concentration in the room does not exceed hazardous levels. In this case, the authors' low cost approach is no longer low cost. Ozone sensors are expensive.  

The duct fan cost around 50 USD, the ozone generator cost around 20 USD, the chassis cost around 20 USD. But each ozone sensor can cost upwards of 150 USD easily. This authors' homemade air purifier would have cost much more than an existing commercial air purifier which is much safer.

Therefore the reviewer cannot be in good conscience endorse a potentially extremely hazardous approach to air purification in the name of unsubstantiated cost reduction after the inclusion of ozone monitors.

(3) However, if the authors wished to re-position their study as a means of a one-time disinfection of an unoccupied building or space, then the authors will need to re-align their study as such. In this case, the authors should be aware that there are already numerous studies on the disinfection efficacy of ozone in unoccupied space for sterilizing airborne viruses (SARS, MERS etc), bacteria and fungi. Many of these studies were performed using ozone generators that are not more costly than what the authors have proposed. Therefore the authors will need a more convincing value proposition for their work.

(4) Commercial passenger airliners employed an inline filtration mechanism where air is first drawn into a chamber to be mixed with ozone to sterilize it. Then the air is passed through a catalyst honeycomb structure to remove the ozone before ejecting the clean air. This design is however already very well known. Like all other air purification systems, the cost of installation is low but the maintenance cost is high.  

Author Response

Dear Reviewer 2

Thank you for your comments on our manuscript. We appreciate the responses and reviewer's opinion. However, we would like to underline that, progress in science, and especially in technology, is based not only on a great discovery, but very often on improving existing technology. Currently, hospitals, clinics and many different industrial companies need a simple, effective and safe air sterilization technique. Ozone generators are well known devices that can be used for this purpose. However, a portable air purifier should be balanced in terms of highly effective sterilization, building residents / workers safety and low costs. We have shown that a real compromise can be achieved in this field. We show in particular that a portable ozone generator can be used safely not only because it has a carbon filter, but also because we have developed a method to calculate the optimal sterilization time.

Aforementioned text could be a comment to remarks 1-3, however, according to the format of revision, the authors should refer separately to each of the reviewer's comments, so below please find our separate comments.

(1) Ozone is a well-known oxidizing and hence sterilizing agent. It is commonly used in the sterilization of water in swimming pools as well as in commercial passenger airliners air sterilization system. Therefore the reviewer's concern regarding this work is not the use of ozone but the manner in which the authors chose to use it.

The design proposed by the authors is to simply eject quantities of ozone into an enclosed space. This is a very dangerous proposition no matter how well intended the authors are. This design is not very different from a gas chamber.

Answer: We agree that such a solution brings risk to the occupants, but comparing it to the gas chamber is an exaggeration. We want to underline that unaware DIY's often use such devices. Our research presents a solution when people want to occupy the room in safe conditions. We proposed a cheap solution which enables a high reduction of bioaerosols in indoor air and a safe entrance of occupants after 1.5 hour.

(2) In order to prevent the authors' design from turning a room into a gas chamber, ozone sensors will have to deployed throughout room as well as adjacent rooms. Only with ozone sensors that is feedback to the ozone generator will ensure that the ozone concentration in the room does not exceed hazardous levels. In this case, the authors' low cost approach is no longer low cost. Ozone sensors are expensive. 

The duct fan cost around 50 USD, the ozone generator cost around 20 USD, the chassis cost around 20 USD. But each ozone sensor can cost upwards of 150 USD easily. This authors' homemade air purifier would have cost much more than an existing commercial air purifier which is much safer.

Therefore the reviewer cannot be in good conscience endorse a potentially extremely hazardous approach to air purification in the name of unsubstantiated cost reduction after the inclusion of ozone monitors.

Answer: Our intension wasn't to encourage people to buy the ozone generator and use it in their home, but we wanted to show if the solution should be safe and effective, it requires both an ozone reduction solution, e.g., carbon filter, and periodical operation to ensure a safe environment for the occupants.

(3) However, if the authors wished to re-position their study as a means of a one-time disinfection of an unoccupied building or space, then the authors will need to re-align their study as such. In this case, the authors should be aware that there are already numerous studies on the disinfection efficacy of ozone in unoccupied space for sterilizing airborne viruses (SARS, MERS etc), bacteria and fungi. Many of these studies were performed using ozone generators that are not more costly than what the authors have proposed. Therefore the authors will need a more convincing value proposition for their work.

Answer: Our intension wasn't to dedicate our device in unoccupied spaces. We wanted to measure the efficiency of a homemade air purifier and check its safety by the use of cheap and available filters.

 (4) Commercial passenger airliners employed an inline filtration mechanism where air is first drawn into a chamber to be mixed with ozone to sterilize it. Then the air is passed through a catalyst honeycomb structure to remove the ozone before ejecting the clean air. This design is however already very well known. Like all other air purification systems, the cost of installation is low but the maintenance cost is high. 

Answer: The research using honeycomb is interesting; we can use such a solution in our future studies.

 Again, we would like to express our appreciation for your efforts and helpful comments. Please find the revised version of our paper enclosed.

Yours sincerely,

Anna Mainka

Reviewer 3 Report

This is certainly an important problem, since, as the authors point out, ozone emissions from these air purifiers often exceed acceptable levels, but consumers are rarely warned about any potential risks.  That said, I have concerns about the novelty of this work.  There are already many papers that have measured ozone emissions by these devices.  (For example: Phillips et al, Journal of Exposure Science & Environmental Epidemiology, 9, 594-601, 1999; Britigan et al, Journal of the Air & Waste Management Association, 56, 601-610, 2006; Boeniger, American Industrial Hygiene Association Journal, 56, 590-598, 2010).  I can support publication of this paper if the authors can do a better job of discussing their work in the context of the existing literature, and show that their work answers unanswered questions. 

Author Response

Dear Reviewer 3

Thank you for your comments on our manuscript. We appreciate the responses and helpful suggestions for improving the presentation of our study. We have followed the comments and marked the text corrections using track changes function.

Below, we have detailed our modifications as they relate to your comments on the original manuscript.

This is certainly an important problem, since, as the authors point out, ozone emissions from these air purifiers often exceed acceptable levels, but consumers are rarely warned about any potential risks. That said, I have concerns about the novelty of this work.  There are already many papers that have measured ozone emissions by these devices.  (For example: Phillips et al, Journal of Exposure Science & Environmental Epidemiology, 9, 594-601, 1999; Britigan et al, Journal of the Air & Waste Management Association, 56, 601-610, 2006; Boeniger, American Industrial Hygiene Association Journal, 56, 590-598, 2010).  I can support publication of this paper if the authors can do a better job of discussing their work in the context of the existing literature, and show that their work answers unanswered questions.

Answer: We have improved the text by adding above mentions papers in the following fragments:

Lines 74-83

As opposed to publications complimenting the ozone-generating devices to improve indoor air quality (IAQ) some studies cite US EPA the Consumers Union statement that “Air cleaners that generate ozone intentionally should not be used indoors” and focus on potentially deleterious consequences of overexposure to ozone as a public health concern [16,27]. For example, Britigan et al. [27] examined thirteen air purifiers and pointed out that ozone generators can produce O₃ levels above public health standards. Except for measurements indoors, in cars or airliner cabins [27,28], the tests with personal air purifiers (PAP) were done [27,29]. All skeptical publications underline that O₃ emission rates can maintain levels over public health standards, particularly in urban areas where ozone levels are already elevated as a result of outdoor emission. They also underline the susceptibility of elderly, children, and persons with chronic diseases to ozone emissions.

 Line 162

… purifiers are characterized by ozone emission rates from 16 mg/h to 220 mg/h [27,46].

Lines 251-253

Britigan et al. [27] pointed that O₃ lifetime is moderately dependent on temperature variation and highly dependent on the presence of many reactive surfaces; for example, inside the car, the lifetime was only 2min.

Lines 390-393

Following Britigan et al. [27] it should be underlined that ozone decrease depends from the surface area of the room, in this case, the loss is dominated by heterogeneous removal on surfaces as well as on the total volume of the room, when the O₃ decrease is dominated by air exchange.

 Again, we would like to express our appreciation for your efforts and helpful comments. Please find the revised version of our paper enclosed.

Yours sincerely,

Anna Mainka

Round 2

Reviewer 2 Report

(1) The authors replied "However, a portable air purifier should be balanced in terms of highly effective sterilization, building residents / workers safety and low costs. We have shown that a real compromise can be achieved in this field. We show in particular that a portable ozone generator can be used safely not only because it has a carbon filter, but also because we have developed a method to calculate the optimal sterilization time."

The reviewer wishes the authors to understand that there is no way that anyone can "calculate" an optimal sterilization time for ozone in a building/room without taking into account the fittings, furnishings, beddings etc. Ozone will react with the furnishings, metal surfaces etc in the building/room. This means more furnishings, metal surfaces, etc in a building/room, less ozone there will be for sterilization. 

Therefore sparsely furnished building/room and a heavily furnished building/room, though having the same size, does not mean that one can sterilize it to the same extent using the same amount for ozone for the same duration. The authors' "calculations" simply did not take the furnishing etc into account.

The only way to figure out whether the ozone has reached sufficient levels to sterilize specific locations of the buildings is to deploy expensive ozone sensors. Otherwise there is no indication whether ozone has build up to a hazardous levels in poor ventilated corners (like stairways etc).  

(2) The authors wrote "We agree that such a solution brings risk to the occupants, but comparing it to the gas chamber is an exaggeration. We want to underline that unaware DIY's often use such devices. Our research presents a solution when people want to occupy the room in safe conditions. We proposed a cheap solution which enables a high reduction of bioaerosols in indoor air and a safe entrance of occupants after 1.5 hour."

As the reviewer has mentioned above, the authors did not take into account furnishings, metal surfaces etc. Without a network of ozone sensors to continuously monitor the ozone levels in different parts of the building/room, it can become a gas chamber. 

(3) The authors replied "Our intension wasn't to dedicate our device in unoccupied spaces. We wanted to measure the efficiency of a homemade air purifier and check its safety by the use of cheap and available filters."

As the reviewer has mentioned above, there is nothing safe about filling a building/room with ozone and have people to go inside after 1.5 hours. And if ozone sensors are needed, it will become very expensive.

(4) The authors wrote "The research using honeycomb is interesting; we can use such a solution in our future studies."

The reviewer is appalled that the authors have never heard of honeycomb structured ozone removal before. The authors should really perform a thorough prior work search before designing and proposing a dangerous approach like their current design. 

What the authors are proposing here is something that engineers have thought of a very long time ago. The reason that it was never implemented is because it is extremely dangerous in absence of real time ozone sensing network. 

As mentioned in the 1st review, in absence of a network of ozone sensors, the reviewer sees this as a very dangerous approach to the sterilization of an actively occupied building/room. Therefore the reviewer will continue to recommend a rejection.

Reviewer 3 Report

I have no objections to the revised paper.