Open letter to SUTD OCIF regarding ozone-generating air cleaner

Here I reproduce my email to SUTD OCIF regarding an ozone-generating air cleaner that has been deployed in my office space. Such devices are marketed base on pseudoscientific claims, which I am committed to combating.

More to come on this.

Hi Yoke Long,

We spoke in-person yesterday about my concerns with the use of an ozone-generating “air cleaning” device (henceforth, “the device”) in a human-occupied office space (Room 1.417). Thank you for being receptive to my comments and agreeing to review additional technical evidence with regards to the safety and efficacy of such deployment of said device.

Here, I briefly restate some key points from our conversation yesterday, as I understand them. The device is marketed as an “air+surface sterilizer” by The Medklinn Group of Companies (henceforth, “the manufacturer”). Specifically, the device in question has model number PRO AS40P. SUTD OCIF has procured and deployed the device in Room 1.417 as part of its strategy to improve indoors air quality (IAQ) in said room — particularly by lowering the elevated formaldehyde levels.

My concern about the deployment of the device is twofold. First, that the device is of questionable efficacy at reducing concentrations of airborne formaldehyde. Second, that the device causes unintended adverse effects injurious to the health of the room’s occupants. I expound on these concerns below.


A useful literature review about the efficacy and safety of ozone-generating air cleaners has been published by the United States Environmental Protection Agency (EPA), with the conclusion that such devices are less effective and carry a higher risk of adverse effects than commonly perceived. This report may be accessed at this link.


In the marketing and technical material accessible by the public from its website, the manufacturer does not provide substantiated evidence for the efficacy of the device in reducing formaldehyde concentrations in indoors air. In fact, the efficacy of ozone-generating air-purifying devices at reducing formaldehyde concentrations have been called into question by (Esswein and Boeniger, 2011). Until and unless the manufacturer is able to provide evidence to the contrary, we must assume that currently available scientific evidence pointing to inefficacy applies to the device.

If the device is less effective at reducing formaldehyde than previously thought, its deployment in Room 1.417 may result in a sense of complacency and failure to pursue alternative formaldehyde-reducing measures aggressively, resulting in unnecessarily prolonged exposure of the room’s occupants to elevated formaldehyde levels.


According to the manufacturer’s website, the device simultaneously generates ozone gas and negative oxygen ions. Ozone is an air pollutant that has been reported to cause respiratory irritation after short-term exposure to relatively low concentrations (0.1-0.2 ppm range), resulting in reduced forced expiratory volume (FEV) and symptoms such as cough and shortness of breath. Effects of long-term exposure are poorly understood and represents an unknown risk. (Lippmann, 1989) provides a review of the literature on the health effects of ozone exposure. It is important to note that adverse health effects were observed at ozone concentrations only slightly above recommended exposure limits.

According to a document published on the manufacturer’s website, the particular device model (PRO AS40P) deployed in Room 1.417 generates 130 mg of ozone per hour and is recommended for use in rooms with 73 square meters of floor area and a 3 m ceiling height. When used according to these manufacturer recommendations, the average concentration of ozone in the room attributable to the device (excluding baseline ozone levels), can be roughly estimated using a conservation-of-mass procedure by considering the ozone-generation rate of the device, the air volume of the room, and the rate of ventilation in terms of room air changes per hour. For reference, the acceptable limit for ozone in indoors air according to the Singapore Standard Code of Practice for Indoor Air Quality for Air-Conditioned Buildings (SS 554:2009), the acceptable limit of ozone is 0.1 ppm. According to the aforementioned recommended use parameters published on the manufacturer’s website, a minimum of 13.5 room air changes per hour is needed to achieve ozone levels below the acceptable limit when the device is turned on. I am doubtful that Room 1.417 achieves such a high rate of air exchange on a continuous basis. In addition, it is important to note that these values correspond to average concentration levels over the entire room, and that the specific pattern of air flow in any given room may result in regions of locally elevated ozone concentration (and corresponding regions of locally depressed ozone concentrations).

In addition to the direct health effects of ozone, recent research has suggested that elevated ozone levels can indirectly worsen IAQ by reacting with other species in the room and generating volatile organic compounds (VOCs) and other oxidation products known to be pollutants (including formaldehyde). This indirect risk is specifically highlighted in SS 554:2009.

In light of the above, I call upon OCIF to review the scientific evidence presented and take steps to quantify the efficacy and safety of the use of the device in Room 1.417 specifically, and as a tool to improve IAQ at SUTD in general. In particular, I recommend OCIF begin monitoring ozone levels alongside formaldehyde levels in Room 1.417 and cease operation of the device until its safety and efficacy has been substantiated.

As I consider this a matter of public interest, and in keeping with principles of openness and transparency, this letter is reproduced verbatim on my personal website and should be considered an open letter.

I am happy to extend any assistance necessary to OCIF as it moves to address this issue. After all, I, as an occupant of Room 1.417, have skin in the game.

You may reach me at this email address, or via telephone at [contact details redacted]

Thank you.

Best regards,

Shien Yang Lee
MIT-SUTD Dual Masters’ Student
Singapore University of Technology and Design (SUTD)
[contact details redacted]