Guide by Vistec Gas Detection. Last updated October 2026.
Nitrogen dioxide (NO2) is a toxic exhaust gas that builds up wherever engines run in enclosed spaces. This guide is for car park operators, facilities managers and M&E designers who want to know where NO2 comes from, what the Irish limits are, and how gas detection is used to control ventilation.
Nitrogen dioxide is a reddish-brown gas with a sharp, acrid smell. It reacts with water to form nitric acid. Together with nitric oxide (NO) it makes up the group of gases known as nitrogen oxides, or NOx.
The EPA explains that nitrogen oxides are released when petrol or diesel is burned in internal combustion engines, and that NO2 is the more important of the two for health.
According to the EPA, the main source of NO2 in Ireland is road transport, and diesel vehicles produce more NO2 than petrol vehicles. Other sources include off-road machinery, construction plant, and boilers and generators. At work, nitrogen dioxide is a concern wherever exhaust gathers in an enclosed or partly enclosed space, such as:
The HSA’s 2026 Code of Practice for the Chemical Agents Regulations sets the Irish occupational exposure limits for nitrogen dioxide at 0.5 ppm as an 8-hour average and 1 ppm over 15 minutes. For comparison, carbon monoxide, the other main exhaust gas, has limits of 20 ppm over 8 hours and 100 ppm over 15 minutes.
The NO2 limits are far lower than the CO limits. A small amount of nitrogen dioxide matters, so the sensor has to be sensitive and well maintained to give a meaningful reading. The Code also makes clear that exposure should be kept well below the limit values and as low as reasonably achievable, not allowed to creep up to them.
Petrol and diesel exhaust do not have the same make-up. Carbon monoxide is the gas Irish ventilation guidance uses to judge car park air, but the EPA notes that diesel engines produce more NO2. A system that only measures CO may not reflect the nitrogen dioxide coming from diesel cars, vans and lorries.
The European standard for this equipment, EN 50545-1, covers fixed apparatus for detecting carbon monoxide, nitric oxide and nitrogen dioxide in car parks and tunnels, including the remote sensors and the control unit. Its scope also extends to similar spaces such as lorry loading areas and underground bus stations. Monitoring CO and NO2 together gives a fuller picture of the air people are breathing.
In Ireland, Technical Guidance Document F of the Building Regulations sets out ventilation for car parks below ground, enclosed car parks and multi-storey car parks. Naturally ventilated car parks need well distributed permanent openings at each level equal to at least 1/20th of the floor area, with at least a quarter of that on each of two opposing walls. Mechanically ventilated car parks are expected to provide either natural openings of at least 1/40th of the floor area with a fan system of at least three air changes per hour, or, for basement car parks, at least six air changes per hour. Exits and ramps where cars queue with engines running need a local rate of at least 10 air changes per hour.
TGD F also allows other systems that limit the mean predicted carbon monoxide level to no more than 30 ppm averaged over 8 hours, with peaks at ramps and exits no higher than 90 ppm for up to 15 minutes. Gas detection is a practical way to manage this. Sensors report to a control panel, fans step up as levels rise and drop back when the air clears, and a higher alarm warns people to leave.
Sensor positions matter as much as sensor type. Honeywell’s technical guide advises mounting detectors where gas is most likely to be present, allowing for natural and forced air currents, and leaving easy access for testing and servicing. Queuing lanes, ramps, pay stations and low corners with poor air movement deserve particular attention.
An addressable system such as the GDS Combi gas range suits larger car parks because each sensor reports separately, so you can see which level or zone is affected. More detail on design and installation is on our car park gas detection page.
The HSA’s 2026 Code of Practice sets 0.5 ppm as an 8-hour average and 1 ppm over a 15-minute period. These are much lower than the carbon monoxide limits of 20 ppm and 100 ppm.
Nitric oxide (NO) and nitrogen dioxide (NO2) are both nitrogen oxides formed when fuel burns in an engine. The EPA notes that NO2 is the more important for health. EN 50545-1 covers detection of both, alongside carbon monoxide.
TGD F sets out fixed ventilation rates, and an alternative route based on keeping carbon monoxide below set levels. Whether detection is part of your design is a decision for your building services designer. Where it is used, it lets fans run on demand rather than continuously.
Diesel engines produce more nitrogen dioxide than petrol engines, according to the EPA. Where diesel vehicles use the space, adding NO2 sensors gives a better picture of the exhaust gases present.
Follow the manufacturer’s service interval. Sensors in car parks face dust, damp and exhaust, so regular calibration with certified test gas keeps readings accurate. A maintenance contract keeps visits on schedule. See our guide on how often gas detectors should be calibrated.
We design, install, commission and maintain CO and NO2 detection that drives car park ventilation. Call 01 289 7720 or email info@vistec.ie.
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