Measurements · Gases & VOCs
Nitric Oxide (NO)
Nitric oxide: a combustion gas and a precursor to NO₂ and ozone.
What it is
What is nitric oxide (NO)?
Nitric oxide is a colorless gas, the first-formed and most abundant of the nitrogen oxides (NOₓ) produced when fuel is burned at high temperature. Fresh from a flame or an exhaust pipe, most NOₓ leaves as NO, which then oxidizes in air to nitrogen dioxide (NO₂). Its sources mirror those of NO₂ (road traffic and diesel engines above all, along with power generation, industrial combustion, and gas appliances), but because NO is what emerges first, it is the more direct marker of a nearby combustion source, before atmospheric chemistry has had time to convert it.
Together, NO and NO₂ make up the NOₓ that drives roadside pollution and feeds the formation of ground-level ozone.
Health and operational effects
At the concentrations found in ambient air, NO is less directly irritating than NO₂, but it matters because it readily oxidizes to NO₂ and because it takes part in the photochemical reactions that generate ozone and secondary particulate. At the higher concentrations possible in industrial and flue-gas settings it becomes toxic in its own right. Measuring NO alongside NO₂ gives the total NOₓ and reveals how fresh an emission is: a high NO-to-NO₂ ratio points to a source close by.
For combustion plant, NO is also a key process parameter: it is a primary flue-gas measurement for tuning burner efficiency and demonstrating NOₓ compliance.
Limits
Nitric Oxide (NO) Limit Values
The published values, by averaging period; each links to the standard that sets it.
8-hour TWA
8-hour TWA (PEL)
NO is reported in ppm or µg/m³. In ambient air it is usually regulated indirectly, as part of total NOₓ or through the NO₂ standards rather than by a limit of its own, while occupational frameworks such as the OSHA PEL and ACGIH TLV do set a workplace value for NO. The applicable values appear in the limit table on this page. Because NO and NO₂ interconvert, the pair is most informative reported together as NOₓ.
How it is measured
How Nitric Oxide (NO) Is Measured
Chemiluminescence analyzers are the reference method, measuring the light emitted when NO reacts with ozone and giving NO directly and NO₂ by difference. For distributed networks, flue-gas analyzers, and lower-cost monitoring, electrochemical sensors produce a current proportional to NO concentration. They are compact and real-time but need compensation for temperature, humidity, and cross-sensitivity to other gases. Aethair PRO can carry a hot-swappable NO gas sensor cartridge, and Thiamis can connect an external multi-gas monitor or dedicated analyzer.
Aethair PRO cartridge1
A pre-calibrated cartridge in one of Aethair PRO's sockets: Aethair NO.
An instrument on Thiamis2
A third-party instrument connected to a Thiamis gateway, reporting into the same Environet account.
Placing NO sensors
For traffic and ambient work, place sensors at the roadside or site boundary at breathing height with the intake clear of obstructions, and record wind speed and direction to relate concentrations to traffic and sources. For combustion process monitoring, NO is sampled in the flue or stack. Because NO oxidizes to NO₂ within minutes of release, the position of the sensor relative to the source strongly affects the NO-to-NO₂ ratio measured, so consistent placement matters when comparing readings over time.
Where it is monitored
Solutions That Monitor Nitric Oxide (NO)
Each one links to the solution page showing what that industry measures it for.
Occupational Safety & Industrial Hygiene
Often addedWorker exposure, confined space, and heat stress
Article Library
Reading on Nitric Oxide (NO)
Ambient Air Quality Monitoring: What Nonattainment Changes
Ambient air quality monitoring explained: how PM2.5 attainment is measured, what a nonattainment designation triggers, and what site-level monitoring adds.
Ozone Exposure Limits and Monitoring: OSHA and NAAQS
OSHA's ozone PEL is 0.1 ppm as an 8-hour TWA. How NIOSH, ACGIH, and the 70 ppb NAAQS compare, and why ozone needs short-interval monitoring.
How to Choose an Air Quality Monitor: PRO, IAQ, or Thiamis
Compare Aethair PRO, IAQ, and Thiamis to match the right device to your site, from commercial interiors to industrial and fenceline monitoring.
OSHA Air Quality Standards 2026: PELs & 29 CFR 1910.1000
OSHA air quality standards for indoor and outdoor work: permissible exposure limits, Table Z-1, 29 CFR 1910.1000, and the heat and wildfire smoke triggers.
Related
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Questions
Nitric Oxide (NO) FAQ
What is the difference between NO and NO₂?
Both are nitrogen oxides from combustion, and together they are called NOₓ. NO is the form emitted first, directly from the flame. It then oxidizes in air to NO₂, the reddish-brown gas that carries the more direct respiratory health effects. Measuring both shows the total NOₓ and how far the conversion has progressed.
Why measure NO as well as NO₂ and ozone?
The three are chemically linked in the photochemical smog system: NO oxidizes to NO₂, which in sunlight helps form ozone, while ozone in turn converts NO back to NO₂. Measuring NO alongside NO₂ and ozone captures this cycle, indicates how fresh an emission is, and gives the total NOₓ that a single measurement would miss.
Is nitric oxide regulated?
In ambient air NO is generally controlled as part of NOₓ or through the NO₂ limits rather than by a standard of its own, so it is often reported together with NO₂. Workplaces do have a specific occupational limit for NO. The applicable values are shown in the limit table on this page.
How do I monitor NO?
Aethair PRO can measure NO with a hot-swappable gas sensor cartridge, or Thiamis can connect an external electrochemical multi-gas monitor or a reference chemiluminescence analyzer. For a full picture of roadside chemistry, combine NO with NO₂ and ozone. Use the configurator to assemble the NOₓ and criteria-pollutant parameters your site needs.
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