If you're serious about indoor air quality, you need to be thinking about ozone monitoring. The risks of ozone exposure have been acknowledged by government agencies including the FDA, OSHA, NIOSH, and EPA, as well as in healthy building standards such as WELL and Fitwel.
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However, ozone is often poorly understood compared to other indoor air quality factors. As a result, companies often struggle to compare between different ozone monitoring solutions, leading to the topic being overlooked or improperly addressed.
In this article, we'll clear up some of the daunting technical terminology around ozone and ozone measurement. By the end, you'll know what ozone is, when you should be concerned about it, and the various types of sensors used to measure ozone levels indoors and outdoors. Are you ready? Let's go!
What is Ozone?
A quick technical definition. Ozone (O3) is a colorless gas composed of three atoms of oxygen. Two oxygen atoms form O2 oxygen, which is essential for all life on Earth. That extra atom can detach from ozone and attach itself to other substances, altering them in the process.
Good vs bad ozone. When ozone occurs naturally in the upper atmosphere, 10 to 30 miles above the ground, it protects us from the sun's ultraviolet radiation that would otherwise have harmful effects on plant and human life. This is the good ozone - the one we're worried is depleting over time (colloquially referred to as the ozone hole).
However, at ground level, ozone is a harmful air pollutant for human beings and the environment. Smog, which most of us have had the displeasure of experiencing, is primarily composed of ozone.
Bad ozone is formed by nitrogen oxides mixing with volatile organic compounds (VOCs), in the presence of sunlight. These compounds are released by burning fossil fuels such as coal and gasoline, as well as by chemicals such as solvents evaporating.
Ozone is a form of oxygen. When formed naturally in the stratosphere, it positively affects life on Earth; when formed at ground level by human activity it is harmful.
For the rest of this article, we'll be referring to ground level or 'bad' ozone.
Why Indoor Ozone is Cause for Concern
In the past, scientists assumed that indoor ozone levels were negligible and not something to worry about. However, today this is seen as a misperception: significant levels of ozone seep indoors from outdoor environments, especially during the summer. We've also filled our offices with more indoor ozone emission devices such as photocopiers, printers, and certain types of air purifiers. Schools and offices have been found to be particularly risky in this regard.
Learn more about common sources of indoor ozone.
Ozone is a health hazard. Inhaling ozone can damage the lungs. At low amounts, this can manifest in chest pain, coughing, shortness of breath and throat irritation. People with existing respiratory conditions can be especially susceptible.
At higher levels and with prolonged exposure, ozone can reduce lung function, inflame and damage cells that line the lungs, aggravate asthma and chronic lung disease, and cause permanent lung damage (see this summary by the EPA).
Ozone can hurt your business. As with other elements of poor indoor air quality, ozone-related ailments hurt businesses over the long run due to the costs of absenteeism and reputational damage. Ozone also plays a part in healthy building standards such as WELL, which may require business to meet thresholds for indoor ozone in order to become certified.
Ways to Measure Ozone - How Ozone Sensors Work
Reducing harmful indoor ozone exposure starts with accurately measuring ozone levels in your buildings. There are two main types of sensors used in commercial air quality monitoring devices:
Metal Oxide Semi-conductor (MOS) Sensors
The nitty-gritty:
MOS sensors work by heating a thin film of metal-oxide particles (or, in the case of Kaiterra products, nanoparticles). When heated to around 300'C, this surface adsorbs ozone and other gasses, releasing electrons from the oxygen present on the surface and changing the electrical resistance of the metal-oxide layer.The sensor detects these changes, which are proportional to the presence of a gas or group of gasses in the air.
Pros:
- Can detect even low levels of ozone typically found outdoors ( < 0.1 ppm)
- Can reach lab levels of accuracy
- Less cross sensitive to nitrogen dioxide
Cons:
- Cross-sensitive to other VOCs typically found indoors
- Takes some time to start up (due to the film needing to heat)
- Require more power than alternatives, which can make buildings less energy efficient
The bottom line:
MOS sensors are accurate and can detect even very low levels of ozone, and aren't likely to report erroneous results due to the presence of NO2, which is often found outdoors. However, they can be cross sensitive to indoor VOCs and can drive up energy costs, which makes them less suitable for indoor air monitoring. This is why Kaiterra uses MOS sensors for monitoring TVOC, but not ozone.
Electrochemical (EC) Sensors
The nitty-gritty:
Simply put, in an electrochemical sensor, ozone gas diffuses across a permeable barrier into a cell containing electrodes and electrolytes. As ozone gas permeates the membrane, it alters the electrochemical potential of the electrodes, increasing the conductivity proportionally to the amounts of ozone in the air. The EC sensor can translate this signal into a numeric ozone concentration (in ppb or ppm).
Contact us to discuss your requirements of commercial ozonator. Our experienced sales team can help you identify the options that best suit your needs.
Pros:
- Return consistent results over time - measurements can be repeated and give an accurate picture of improvement or decline over time
- Accurate at detecting ozone at the ppm levels that are typically found indoors
- Less susceptible to cross-interference from other volatile organic compounds (VOCs)
Cons:
- Less accurate at detecting lower ozone levels, which might be needed for ambient ozone measurement)
- Can be susceptible to changes in humidity
- Cross sensitive to nitrogen dioxide (NO2) - typically generated by diesel engines and present outdoors
The bottom line:
Electrochemical sensors are the go-to solution for measuring ozone indoors, where they can provide accurate and consistent measurements and clearly 'single out' ozone among other VOCs. This is why Kaiterra uses EC sensors for monitoring ozone using the Sensedge Mini.
Challenges of Measuring Ozone
Despite increased awareness of the risks of indoor exposure to ozone, many challenges still remain when it comes to measuring it accurately:
Lack of Consistent Standards
Different government agencies and certifying organizations have set different thresholds for acceptable levels of ozone, for example:
- According to the US Occupational Safety and Health Administration (OSHA), workers should not be exposed to more than 0.10 ppm over a period of 8 hours.
- The UK's Health and Safety Executive (HSE) set the workplace exposure limit at 0.2 ppm over a 15 minute reference period.
- The WELL v2 standard talks about 0.051 ppm.
- Fitwel's Enhanced Indoor Air Quality Testing and Monitoring Protocol sets the level at 0.07 ppm
Despite increased awareness of the risks of indoor exposure to ozone, many challenges still remain when it comes to measuring it accurately:Different government agencies and certifying organizations have set different thresholds for acceptable levels of ozone, for example:
As we've covered in our previous article about ozone in the workplace, there's no definitive way to frame indoor ozone, and instead the tendency is to rely on industrial exposure limits and ambient ozone breakpoints. These environments are not necessarily identical to an office.
Variable Ozone Levels Make Measurement Difficult
Another major challenge in measuring ozone is that it varies dramatically. As Charles Weschler summarizes in Ozone in Indoor Environments: Concentration and Chemistry:
Under normal conditions, the half-life of ozone indoors is between 7 and 10 min and
is determined primarily by surface removal and air exchange.
Indoor ozone concentrations can vary from hour-to-hour, day-to-day, and season-to-season, as well as from room-to-room and structure-to-structure... It is not unusual for indoor ozone levels to change 30 or 40 ppb in less than 1 hour. Given that indoor ozone levels vary with multiple factors including time-of-day, weather conditions, ventilation parameters, geographic location and season, it is not reasonable to speak of a single indoor ozone concentration that is representative of most indoor environments.
These frequent and unpredictable changes in ozone levels mean that measurement needs to be an ongoing process rather than a singular effort. Organizations looking to effectively implement ozone monitoring will find that continuous monitoring is better suited, rather than periodic spot testing (read our full comparison between the two).
Incorporate Ozone Monitoring in Your Indoor Air Quality Strategy
As you become more familiar with different aspects of indoor air quality, you can better tailor your tools and strategies. Getting a comprehensive picture of the current state of air quality in your buildings - including ozone, particular matter, variants of concern and carbon dioxide levels - is an essential step towards improving it.
Ready to get started? We've recently announced that Kaiterra's Sensedge Mini indoor air quality monitor now supports ozone monitoring. Check out the details below:
Ozone meter is the principle of ultraviolet absorption method, with stable ultraviolet light source to produce ultraviolet light, with light wave filter out other wavelengths of ultraviolet light, only allow wavelength 253.7nm through. After passing through the sample photoelectric sensor, and then through the ozone absorption tank, it reaches the sampling photoelectric sensor. By comparing the electrical signals of the sample photoelectric sensor with those of the sample photoelectric sensor, and then calculating the mathematical model, the ozone concentration can be obtained.
Ozone is extremely chemically active. Its energy when free produces strong oxidation in an instant, which has the effect of sterilization, disinfection and detoxification. It is widely used for sterilization of water plants, disinfection of tableware, indoor air purification, preservation of fruits and vegetables, and medical treatment. However, if the concentration of ozone is too high, it will cause great harm to human health. Therefore, whether to take advantage of the strong oxidizing properties of ozone or to prevent its harmful effects, the monitoring of ozone concentration is indispensable.
AO3 Detector Dissolved Ozone Meter
What is an ozone meter?
Ozone meter is the most effective method of detecting ozone concentrations. It is a device that converts information such as the composition and concentration of ozone gas in the environment (Air or water) into information that can be used by personnel, instruments, computers, etc.
Principle of ozone meter
The ozone sensor is the core component of the ozone meter, which is in direct contact with the measured gas and converts it into an electrical signal. According to the different principles of detection, ozone meters can generally be divided into three kinds of ultraviolet, semiconductor, electrochemical.
Ultraviolet ozone meter working principle
Research shows that the 253.7nm UV light of ozone has a large absorption coefficient, and the UV light of this wavelength is attenuated by ozone, in accordance with Lambert-Bier's law. The ozone gas meter uses the principle of UV absorption. It uses a stable UV light source to generate UV light. The filter filters out other wavelengths of UV light, allowing only the 253.7 nm UV light to pass through. After passing through the sample photoelectric sensor, it is absorbed by the ozone to reach the sample photoelectric sensor, compare the electrical signals from the sample photoelectric sensor and the sample photoelectric sensor, and obtain the ozone concentration according to the Lambert-Beer method.
Semiconductor ozone meter working principle
Semiconductor ozone sensors use gas-sensitive semiconductor materials, such as WO3, Sn0, In2O3 and other oxide sheets made from sensitive elements. When they absorb ozone, a redox reaction occurs, generating or emitting heat, causing a corresponding change in element temperature and a change in resistance. The ozone concentration is converted into an electrical signal to measure the ozone concentration. Generally as the elevated concentration increases, the resistance of the element increases significantly and is linear over a range.
Electrochemical ozone meter working principle
The electrochemical ozone sensor consists of a working electrode, counter electrode, participating electrode, electrolyte and circuit system. A constant potential value can be maintained between the working electrode and the participating electrode. When the ozone diffuses into the sensor, the reduction reaction occurs at the working electrode and the oxidation reaction occurs at the electrode, forming a tiny current between the electrovalence and the working electrode, and this current enters the sensor with an ozone concentration proportional to a certain range, which is finally processed by the circuit system to calculate the ozone content.
How to choose an ozone meter?
When buying an ozone meter, first of all, we should clarify the characteristics of the use environment and the purpose of use, and choose the ozone monitoring equipment according to its stability, sensitivity, selectivity, corrosion resistance, etc.
- Stability refers to the stability of the basic response of the sensor throughout the working time, depending on the zero drift and interval drift. Zero drift refers to the change in sensor output response throughout the operating time in the absence of ozone. Interval drift is the change in the output response of a sensor placed continuously in ozone and is expressed as a drop in the sensor output signal over the operating time. Ideally, for sensors under continuous operating conditions, the annual zero drift is less than 10%.
- Sensitivity is the ratio of the sensor output change to the measured input change, mainly depends on the technology used in the sensor structure. Most gas sensors are designed using biochemical, electrochemical, physical and optical principles. The first consideration is to select a sensitive technology with sufficient sensitivity to detect the percentage of the threshold limit (TLV ' threshold limit value) or the lowest explosive limit (LEL ' lower explosive limit) of the target gas.
- Selectivity is also known as cross-sensitivity. It can be determined by measuring the sensor response produced by a certain concentration of interfering gas. This response is equivalent to the sensor response produced by a certain concentration of ozone. This characteristic is important in applications where multiple gases are tracked because cross-sensitivity reduces the repeatability and reliability of the measurement. The ideal sensor should have high sensitivity and high selectivity.
- Corrosion resistance refers to the ability of the sensor to be exposed to high volume fraction target gases. The probe should be able to withstand 10-20 times the expected gas volume fraction when a large amount of gas is leaking. Under normal operating conditions, sensor drift and zero calibration values should be as small as possible.
Applications
- Environmental monitoring: Monitoring ozone concentration in the atmosphere to assess air quality.
- Water Treatment: Monitoring ozone concentration in drinking water and industrial water to ensure sterilization.
- Food Processing: Monitor ozone concentration in food processing to ensure food safety.
- Healthcare: Monitor ozone concentration in hospital operating rooms and wards to provide a safe medical environment.
- Industrial production: Monitor ozone leakage during industrial production to ensure production safety.
Future development of ozone sensors
- Miniaturization and integration: smaller size, easier to integrate into various devices.
- Intelligent: with self-calibration, self-diagnosis and other functions.
- Multifunctionality: Simultaneous detection of multiple gases.
- Low-cost: Lower production costs and wider availability.