Nitrogen oxide emissions (NO and NO₂): lowering of the thresholds as of 1st July 2020!
Some of the eight new STELs laid down in the French decree dated 27/12/2019 concern nitrogen oxides (NOx), of which the most well known are NO and NO₂. Their exposure limits will now be stricter: for NO, the threshold has dropped from 25ppm to 2ppm (8-hour STEL) and for NO₂, from 1 to 0.5ppm (15-minute STEL).
These thresholds will apply as of 1st July 2020, which is why we need to prepare for them now, to ensure we are in conformity with the European directive. Click here to read the decree.
Description of nitrogen oxides: NOx (NO and NO₂)
The nitrogen oxide family (or NOx) mainly includes two molecules: nitric oxide NO and nitrogen dioxide NO₂. They are highly irritant gases with a pungent odour in low concentrations and a yellowish to reddish-brown colour. Nitric oxide is produced during combustion at high temperatures by oxidation of the nitrogen contained in the air. Nitrogen dioxide (NO₂) results from its oxidation in the atmosphere. These molecules are widely known because of their role in the formation of ozone (a greenhouse gas) and are partly responsible for acid rain.
There are three ways in which nitrogen oxides form and, consequently, three types of NOx:
- Thermal NOx, formed by the chemical reaction between oxygen and the nitrogen found in the air during combustion at very high temperatures
- Combustible NOx, produced by oxidation of the oxygen present in fuel
- Early-stage NOx, formed by a chemical reaction between the nitrogen found in the air and hydrocarbon radicals (CH and CH₂, for example), which react again with the oxygen in the air.
To find out more about nitric oxide, please read through our gas guide: NO, NO₂
Sources of NOx emissions (NO and NO₂)
The main sources of nitrogen oxide (NOx) emissions are related to human activities, primarily processes performed at very high temperatures.
Measures taken since the year 2000 to cut emissions caused by road transport and fixed installations (power stations, heating systems, etc.) have proved effective. It is therefore essential to continue efforts to reduce the emissions produced by fixed sources.
To sum up, we find nitrogen oxides (nitric oxide and nitrogen dioxide) in:
- The air in cities and industrial areas: near major roads as well as in urban areas.
- Inside buildings (especially houses) that use gas appliances such as cookers, water heaters, oil and petrol stoves, wood-burning stoves, etc.
- Cigarette smoke
- Some work environments involving the production, reduction and handling of nitric acid, metal stripping using acid; production of chemical substances; welding activities (arc welding and welding with a blowtorch); natural gas combustion in the presence of oxygen
- Silos for cereals (decomposition of nitrites and nitrates)
Effects of NOx (NO and NO₂) on health and the environment
1 – Impacts of NOx on health
NOx affect our health. NO₂ is an irritant gas that gets into our lungs. It can lead to breathing difficulties or bronchial hyperreactivity in sensitive populations such as children, older people and people suffering from other pathologies (asthma, etc.).
NO₂ is 40 times more toxic than carbon monoxide (CO) and four times more toxic than NO.
2 – Impacts of NOx on the environment
Associated with Volatile Organic Compounds (VOCs), and under the effect of the sun’s rays, NOx contribute to the formation of ozone in the lower layers of the Earth’s atmosphere (troposphere). NOx are also involved in the acid rain phenomenon, and the build-up of fine particles in the atmosphere.
The quantity of NO₂ in the air is monitored. The alert threshold is 200µg/m³. In France, the threshold values are still sometimes exceeded, but not so frequently as they used to be. According to the French Ministry for Sustainable Development, NOx emissions dropped by 49% between 2000 and 2016. The highest concentrations of NO₂ in France are found in and around the three largest cities: Paris, Lyon and Marseille.
Nitrogen oxide poisoning (NOx)
Effects of nitric oxide NO
To our knowledge, there have been no cases of serious poisoning by NO. When there is a high concentration of NO, haemoglobin transforms into methaemoglobin (Met-hb), reducing the amount of oxygen that can be carried by haemoglobin. Given the concentrations found in the atmosphere, NO has no harmful effects and can actually be beneficial for the respiratory function!
For several years already, NO has been used to improve resuscitation outcomes at Lariboisière hospital in Paris, with relatively high concentrations. When inhaled in concentrations of between 30 and 100mg per cubic metre of air, NO improves vasodilatation and lowers pulmonary blood pressure. It has a positive effect on Acute Respiratory Distress Syndrome (ARDS) because it raises oxygen levels in the blood by improving gas exchanges in the lungs. In the case of ARDS, NO also enables us to reduce inhalation of pure oxygen, which is toxic for the lungs.
To sum up, the evidence suggests that nitrogen oxides, at the concentration levels found in the atmosphere, have no harmful effect on the respiratory function. However, when combined with other atmospheric pollutants, especially under the effect of the sun’s rays, reactions can take place that lead to the creation of secondary compounds in concentrations that are sufficient to produce adverse effects.
For information: tests performed with concentrations of 3mg of NO per cubic metre of air (much higher concentrations than the maximum concentrations found in the atmosphere) showed that there was no formation of Met-hb.
When should you protect yourself against NO₂, NO?
In road tunnels
Nitrogen dioxide (NO₂) is a pollutant released by the exhaust pipes of road vehicles. It is considered to be toxic by the WHO and the public authorities, because it is a secondary particulate matter precursor (ultrafine particles formed after the exhaust fumes are released). This is why NO₂ exposure is regulated in road tunnels and the atmosphere in specific conditions that pose a risk (traffic congestion, limited renewal of natural air).
For the past thirty years or so, road tunnels have been progressively equipped with NOx detection devices that help control ventilation. The first devices installed were almost always NO electrochemical sensors. They were gradually replaced by NO₂ detectors, as better technology became available.
Today, the advice is not to use NO electrochemical sensors, replacing them with NO₂ electrochemical sensors both in new installations and when renewing existing ones.
In silos for grain storage
Plants use nitrates in order to grow. During silage* of forage, nitrates are transformed into nitrogen oxides. A peak in the concentration of nitrogen dioxide (29ppm) occurs 12 to 60 hours after silage. It may be influenced by:
- Harvesting after heavy rain occurring after a period of drought
- Reduced photosynthesis
- Excessive fertilisation
The first symptoms can start to appear at 1ppm. The concentrations then begin to drop gradually: the gases are dangerous for 2-3 weeks and sometimes up to 6 weeks after silage. Exposure occurs when you open the door or go into the silo. A poorly ventilated silo can increase the risk.
*Definition of silage: preservation process for fresh forage such as oats, maize and lucerne by partial fermentation in a more-or-less airtight container such as a silo.
Interesting info:
Canadian law requires farmers to follow a comprehensive procedure before entering a silo. To prevent accidents caused by an oxygen-depleted atmosphere in silage silos, CNESST requires farmers to follow a full procedure before entering the closed area of a silo. CNESST clearly states that “if the gas detection readings (O₂, CO₂, NO, NO₂) do not confirm the atmosphere is safe, it is mandatory to wear breathing apparatus when you go into a silo.”
In rocket launch areas
In the aerospace industry, NO₂ comes from the combustive dinitrogen tetroxide, commonly referred to as nitrogen tetroxide, with the chemical formula N₂O₄. A powerful oxidizer which is both corrosive and toxic, it is a colourless or brownish-yellow liquid depending on the concentration of nitrogen dioxide (NO₂) it contains. It is a dimer of N₂O₄.
More commonly known as NTO (nitrogen tetroxide), dinitrogen tetroxide is one of the main propellants used today as rocket fuel. It replaced nitric acid in the 1990s. For Ariane, Soyuz and VEGA, specific mixtures are used to operate the combustion chambers:
- Nitrogen tetroxide and hydrazine
- Nitrogen tetroxide and kerosine (Sozouz)
This compound is often found in rocket launch areas, satellite preparation halls and liquid propellant storage areas.
How can we detect the presence of nitric oxide or nitrogen dioxide as per the new decree of 27/12/2019?
The new decree of 27/12/2019 drastically lowered the STEL thresholds for the nitrogen oxides NO and NO₂, and the new thresholds will be applicable as of 1st July 2020.
- For NO, the threshold drops from 25ppm to 2ppm (8-hour STEL)
- For NO₂, the threshold drops from 1 to 0.5ppm (15-minute STEL)
Dräger, which produces its detectors in full, right down to the sensors, to ensure it has 100% control over the process, offers a NO₂ LC sensor (Dräger USP) which is capable of monitoring such low thresholds. The manufacturer has successfully developed electrochemical sensors that are very reliable over time, and also offer lower operating costs.
DrägerSensor XXS sensors
The portable gas detection devices used in everyday contexts need to be as compact, lightweight and robust as possible, while offering excellent performance levels. For Dräger, this was reason enough to significantly reduce the volume and weight of its sensors and develop a new generation of miniature XXS sensors. Thanks to their very high sensitivity, improved gas selectivity, excellent long-term stability and fast response times, users can rest assured that they will be warned quickly and reliably of the presence of any dangerous gas – a significant boost to their safety in the workplace.
These sensors for the detection of nitrogen oxides NO and NO₂ are compatible with portable detectors: PAC 8000 and X-AM 5000/5600 ranges.
In addition to gas detection, Dräger offers respiratory protection solutions: the Saver CF, a constant flow emergency escape breathing apparatus. Easy to put on and automatic, it allows safe, effective and uncomplicated escape from hazardous environments.
