Should You Monitor Nitrogen Dioxide (NO2) in the Workplace?

If you’re familiar with indoor air quality (IAQ) monitoring, then you probably know some of the major indoor air parameters that you should measure: particulate matter, CO2, volatile organic compounds, etc. These parameters exist in every building and space, and should therefore be monitored continuously to ensure they stay at healthy levels.

However, there are other indoor air parameters that you may not be as familiar with — parameters that, despite not existing in every building and every space, are common enough that you should still be on the lookout for them in your workplace. One of these parameters is nitrogen dioxide (NO2), and that’s what we’ll be talking about in this article, starting with the basics: understanding what NO2 is and why it’s important to monitor in the indoor environment.

What Is NO2?
NO2 is a gaseous byproduct that forms when fossil fuels (such as coal, oil, methane, and diesel) are burned to run vehicles, power plants, and a wide range of industrial processes. 

Like many other air pollutants, NO2 is known for its harmful effects on human health. Short-term exposure to high NO2 levels can cause respiratory issues (everything from coughing and wheezing to inflamed airways and headaches) while long-term exposure can impair lung function, increase the risk or severity of asthma, interfere with your ability to fight off respiratory infections, and even lead to respiratory emergencies.

Needless to say, NO2 poses a significant threat to human health, especially with consistent, daily exposure (even at low levels). Most people recognize NO2 as an outdoor air pollutant; however, it can easily infiltrate indoor spaces, especially in areas where outdoor NO2 levels are high, like urban areas or places close to roads with heavy traffic. This means that countless people are at risk for NO2 exposure every day when they go to work, even if they work in an office building doing a job that (technically) has no occupational exposure to NO2.

What Are Sources of NO2 in the Workplace?

Unless your workplace has a combustion appliance (gas stove, furnace, water heater, etc.), you typically won’t see NO2 coming from the indoor environment itself. However, just like all other air pollutants, high outdoor levels of NO2 can easily seep inside and increase indoor concentrations beyond a healthy threshold.

If your workplace is located in a city, near major highways, or close to heavy industrial activity (like a power plant), there’s a good chance that outdoor NO2 levels may be elevated — and, if they are, this NO2 can get into your building through doors, windows, and even your ventilation system.

Buildings with enclosed or underground parking lots are also at risk for high indoor NO2 levels. When vehicles idle in these spaces, they release toxic fumes with high amounts of NO2. Usually, the attached buildings are designed so that the ventilation from the parking lot can’t get inside, but in reality, many experience issues with the design, operation, or maintenance of their HVAC systems, so it’s not uncommon to detect NO2 seeping into the rest of the building from these spaces.

Even if your building doesn’t have an enclosed parking lot and isn’t located in a major urban area, the risk of excess indoor NO2 is not always zero. Any proximity to NO2-emitting vehicles or objects can raise your indoor NO2 beyond a healthy level, including generators, delivery docks, maintenance bays, train tracks, etc.

“A WELL consultant recently told us about a project in Australia that failed its NO2 performance test, which was incredibly confusing to everyone because the building had no known sources of NO2. After some digging, they found that on the day of the test, a diesel train happened to be passing by and idled right next to the building. This, of course, increased outdoor NO2 levels, which then got into the building and increased indoor NO2 levels, causing them to fail their test. This story goes to show just how easily NO2 can get into our buildings and impact our workplace health and well-being, which is why we should be monitoring these levels continuously!”

- Liam Bates, Kaiterra CEO

The key takeaway for any workplace leader or building manager is this: even if you don’t see any clear sources of NO2 in your building, it could still be accumulating inside from any number of outdoor sources, but there’s no way to know unless you continuously measure levels inside your building.

Regulatory Guidelines for Outdoor and Indoor NO2 Exposure

Because NO2 is a well-understood outdoor air pollutant, several regulatory bodies have established standards for safe levels of exposure. While these standards specifically apply to outdoor air, they provide good reference points that buildings can use to evaluate their indoor levels:

• The U.S. Environmental Protection Agency (EPA) recommends a 1-hour average limit of 100 parts per billion (ppb) and an average annual limit of 53 ppb of NO2.
• The World Health Organization (WHO) provides the most stringent guidelines, recommending a 24-hour average limit of 25 µg/m³ (approximately 13.3 ppb), with an average annual limit of 10 µg/m³ (approximately 5.3 ppb).
• The Occupational Safety and Health Administration (OSHA) defines a permissible exposure limit (PEL) of 5 ppm (5000 ppb) of NO2 over an average 8-hour workday.
• The National Institute for Occupational Safety and Health (NIOSH) has a recommended exposure limit (REL) of 1 ppm (1000 ppb) for short-term exposure.

While not a regulatory standard, the WELL v2 Building Standard provides one of the only guidelines for indoor NO2 levels. This threshold, found in Feature A05, is 21 ppb (40 µg/m³) or lower. As you can see, this threshold is lower than in most regulatory guidelines (except for the WHO, which recently lowered its limit by 75% following new data indicating the far-reaching impacts of NO2 on human health). 

If you’re currently monitoring or plan to start monitoring NO2 in your building, then we recommend aligning with WELL’s NO2 threshold of 21 ppb (40 µg/m³), given the standard’s evidence-based guidelines for healthy indoor environments.

Best Practices for Monitoring NO2 in the Workplace

Since NO2 is not as ubiquitous as PM2.5 or CO2, many people ask whether or not they should install NO2 sensors at the same density as sensors for these other parameters, especially if their building has a low risk of NO2 pollution. At Kaiterra, these are the four best practices we recommend:

1. Before getting started, it’s important to note that not all air quality monitors offer NO2 sensors, so make sure to choose a solution like the Kaiterra Sensedge Go that can bundle all of these parameters into one monitor (so you don’t have to install another device to get NO2 readings).
2. If you have any risk of high NO2 in your building, whether from nearby traffic or industrial activity, then we recommend having NO2 sensors in each IAQ monitor you install. If you’re just getting started monitoring your IAQ, read more about coverage areas, monitor density, and placement guidelines.
3. If you’re at a lower risk of having NO2 in your building, we still recommend monitoring it so you can find and troubleshoot any unexpected NO2 spikes. However, you probably don’t need to have NO2 sensors installed in every single room. Our rule of thumb is to have at least one NO2 sensor per HVAC zone to make sure you’re getting a clear enough picture of NO2 levels across your entire building or space.
4. Track your NO2 levels continuously to identify patterns and determine whether or not you should change your strategy, whether it’s increasing your monitor density, identifying the source(s) of NO2 events, examining your HVAC system’s efficiency, or adjusting your outdoor air intake timing (if NO2 is getting in from outside).

No matter your situation, it’s always best to work with an IAQ monitoring expert to evaluate your NO2 risk and come up with a plan to ensure you maximize your monitoring coverage, gain full visibility into your building’s performance, and, most importantly, make sure your occupants are working and breathing in a healthy indoor environment.

If you’d like to connect with an expert, reach out today, and a member of our team will be happy to create a tailored NO2 monitoring strategy for your building.