Nitrogen dioxide
Skeletal formula of nitrogen dioxide with some measurementsEP
Skeletal formula of nitrogen dioxide with some measurementsEP
Spacefill model of nitrogen dioxide
Spacefill model of nitrogen dioxide
Nitrogen dioxide at different temperatures
converts to the colorless dinitrogen tetroxide (N
) at low temperatures and reverts to NO
at higher temperatures.
IUPAC name
Nitrogen dioxide
Other names
Nitrogen(IV) oxide,[1] deutoxide of nitrogen
3D model (JSmol)
ECHA InfoCard 100.030.234 Edit this at Wikidata
EC Number
  • 233-272-6
RTECS number
  • QW9800000
UN number 1067
  • InChI=1S/NO2/c2-1-3 checkY
  • InChI=1/NO2/c2-1-3
  • N(=O)[O]
  • [N+](=O)[O-]
Molar mass 46.005 g·mol−1
Appearance Brown gas[2]
Odor Chlorine-like
Density 1.880 g/L[2]
Melting point −9.3 °C (15.3 °F; 263.8 K)[2]
Boiling point 21.15 °C (70.07 °F; 294.30 K)[2]
Solubility Soluble in CCl
, nitric acid,[3] chloroform
Vapor pressure 98.80 kPa (at 20 °C)
+150.0·10−6 cm3/mol[4]
1.449 (at 20 °C)
37.2 J/(mol·K)
240.1 J/(mol·K)
+33.2 kJ/mol
Occupational safety and health (OHS/OSH):
Main hazards
Poison, oxidizer
GHS labelling:
GHS03: Oxidizing GHS04: Compressed Gas GHS05: Corrosive GHS06: Toxic GHS08: Health hazard
H270, H314, H330
P220, P260, P280, P284, P305+P351+P338, P310
NFPA 704 (fire diamond)
Lethal dose or concentration (LD, LC):
30 ppm (guinea pig, 1 h)
315 ppm (rabbit, 15 min)
68 ppm (rat, 4 h)
138 ppm (rat, 30 min)
1000 ppm (mouse, 10 min)[7]
64 ppm (dog, 8 h)
64 ppm (monkey, 8 h)[7]
NIOSH (US health exposure limits):
PEL (Permissible)
C 5 ppm (9 mg/m3)[6]
REL (Recommended)
ST 1 ppm (1.8 mg/m3)[6]
IDLH (Immediate danger)
13 ppm[6]
Safety data sheet (SDS) ICSC 0930
Related compounds
Dinitrogen pentoxide

Dinitrogen tetroxide
Dinitrogen trioxide
Nitric oxide
Nitrous oxide

Related compounds
Chlorine dioxide
Carbon dioxide
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
☒N verify (what is checkY☒N ?)

Nitrogen dioxide is a chemical compound with the formula NO2. One of several nitrogen oxides, nitrogen dioxide is a reddish-brown gas. It is a paramagnetic, bent molecule with C2v point group symmetry. Industrially, NO2 is an intermediate in the synthesis of nitric acid, millions of tons of which are produced each year, primarily for the production of fertilizers.

Nitrogen dioxide is poisonous and can be fatal if inhaled in large quantities.[8] The LC50 (median lethal dose) for humans has been estimated to be 174 ppm for a 1-hour exposure.[9] It is also included in the NOx family of atmospheric pollutants.


Nitrogen dioxide is a reddish-brown gas with a pungent, acrid odor above 21.2 °C (70.2 °F; 294.3 K) and becomes a yellowish-brown liquid below 21.2 °C (70.2 °F; 294.3 K). It forms an equilibrium with its dimer, dinitrogen tetroxide (N2O4), and converts almost entirely to N2O4 below −11.2 °C (11.8 °F; 261.9 K).[6]

The bond length between the nitrogen atom and the oxygen atom is 119.7 pm. This bond length is consistent with a bond order between one and two.

Unlike ozone (O3) the ground electronic state of nitrogen dioxide is a doublet state, since nitrogen has one unpaired electron,[10] which decreases the alpha effect compared with nitrite and creates a weak bonding interaction with the oxygen lone pairs. The lone electron in NO2 also means that this compound is a free radical, so the formula for nitrogen dioxide is often written as NO2.

The reddish-brown color is a consequence of preferential absorption of light in the blue region of the spectrum (400–500 nm), although the absorption extends throughout the visible (at shorter wavelengths) and into the infrared (at longer wavelengths). Absorption of light at wavelengths shorter than about 400 nm results in photolysis (to form NO + O, atomic oxygen); in the atmosphere the addition of the oxygen atom so formed to O2 results in ozone.


See also: Ostwald process

Industrially, nitrogen dioxide is produced and transported as its cryogenic liquid dimer, dinitrogen tetroxide. It is produced industrially by the oxidation of ammonia, the Ostwald Process. This reaction is the first step in the production of nitric acid:[11]

4 NH3 + 7 O2 → 4 NO2 + 6 H2O

It can also be produced by the oxidation of nitrosyl chloride:

2 NOCl + O2 → 2NO2 + Cl2

Instead, most laboratory syntheses stabilize and then heat the nitric acid to accelerate the decomposition. For example, the thermal decomposition of some metal nitrates generates NO2:[12]

Pb(NO3)2 → PbO + 2 NO2 + 12 O2

Alternatively, dehydration of nitric acid produces nitronium nitrate...

2 HNO3 → N2O5 + H2O
6 HNO3 + 12 P4O10 → 3 N2O5 + 2 H3PO4

...which subsequently undergoes thermal decomposition:

N2O5 → 2 NO2 + 12 O2

NO2 is generated by the reduction of concentrated nitric acid with a metal (such as copper):

4 HNO3 + Cu → Cu(NO3)2 + 2 NO2 + 2 H2O

Selected reactions

Nitric acid decomposes slowly to nitrogen dioxide by the overall reaction:

4 HNO3 → 4 NO2 + 2 H2O + O2

The nitrogen dioxide so formed confers the characteristic yellow color often exhibited by this acid. However, the reaction is too slow to be a practical source of NO2.

Thermal properties

At low temperatures, NO2 reversibly converts to the colourless gas dinitrogen tetroxide (N2O4):

2 NO2 ⇌ N2O4

The exothermic equilibrium has enthalpy change ΔH = −57.23 kJ/mol.[13]

At 150 °C (302 °F; 423 K), NO2 decomposes with release of oxygen via an endothermic process (ΔH = 14 kJ/mol):

2 NO2 →2 NO +  O2

As an oxidizer

As suggested by the weakness of the N–O bond, NO2 is a good oxidizer. Consequently, it will combust, sometimes explosively, in the presence of hydrocarbons.[14]


NO2 reacts with water to give nitric acid and nitrous acid:

2 NO2 + H2O → HNO3 + HNO2

This reaction is one of the steps in the Ostwald process for the industrial production of nitric acid from ammonia.[11] This reaction is negligibly slow at low concentrations of NO2 characteristic of the ambient atmosphere, although it does proceed upon NO2 uptake to surfaces. Such surface reaction is thought to produce gaseous HNO2 (often written as HONO) in outdoor and indoor environments.[15]

Conversion to nitrates

NO2 is used to generate anhydrous metal nitrates from the oxides:[13]

MO + 3 NO2 → M(NO3)2 + NO

Alkyl and metal iodides give the corresponding nitrates:[10]

TiI4 + 8 NO2 → Ti(NO3)4 + 4 NO + 2 I2

With organic compounds

The reactiivity of nitrogen dioxide toward organic compounds has long been known.[16] For example, it reacts with amides to give N-nitroso derivatives.[17] It is used for nitrations under anhydrous conditions.[18]


NO2 is used as an intermediate in the manufacturing of nitric acid, as a nitrating agent in the manufacturing of chemical explosives, as a polymerization inhibitor for acrylates, as a flour bleaching agent,[19]: 223  and as a room temperature sterilization agent.[20] It is also used as an oxidizer in rocket fuel, for example in red fuming nitric acid; it was used in the Titan rockets, to launch Project Gemini, in the maneuvering thrusters of the Space Shuttle, and in uncrewed space probes sent to various planets.[21]

Environmental presence

Nitrogen dioxide tropospheric column density in 2011.

Nitrogen dioxide typically arises via the oxidation of nitric oxide by oxygen in air (e.g. as result of corona discharge):[13]

2 NO + O2 → 2 NO2

NO2 is introduced into the environment by natural causes, including entry from the stratosphere, bacterial respiration, volcanos, and lightning. These sources make NO2 a trace gas in the atmosphere of Earth, where it plays a role in absorbing sunlight and regulating the chemistry of the troposphere, especially in determining ozone concentrations.[22]

Anthropogenic sources

Nitrogen dioxide diffusion tube for air quality monitoring in the City of London.

Nitrogen dioxide also forms in most combustion processes. At elevated temperatures nitrogen combines with oxygen to form nitrogen dioxide:

N2 + 2 O2 → 2 NO2

For the general public, the most prominent sources of NO2 are internal combustion engines, as combustion temperatures are high enough to thermally combine some of the nitrogen and oxygen in the air to form NO2.[8]

Outdoors, NO2 can be a result of traffic from motor vehicles.[23] Indoors, exposure arises from cigarette smoke,[24] and butane and kerosene heaters and stoves.[25] Indoor exposure levels of NO2 are, on average, at least three times higher in homes with gas stoves compared to electric stove.[26][27]

A "fox tail" over Nizhniy Tagil Iron and Steel Works

Workers in industries where NO2 is used are also exposed and are at risk for occupational lung diseases, and NIOSH has set exposure limits and safety standards.[6] Workers in high voltage areas especially those with spark or plasma creation are at risk.[citation needed] Agricultural workers can be exposed to NO2 arising from grain decomposing in silos; chronic exposure can lead to lung damage in a condition called "silo-filler's disease".[28][29]


Main article: Nitrogen dioxide poisoning

NO2 diffuses into the epithelial lining fluid (ELF) of the respiratory epithelium and dissolves. There, it chemically reacts with antioxidant and lipid molecules in the ELF. The health effects of NO2 are caused by the reaction products or their metabolites, which are reactive nitrogen species and reactive oxygen species that can drive bronchoconstriction, inflammation, reduced immune response, and may have effects on the heart.[30]

Acute exposure

Acute harm due to NO2 exposure is rare. 100–200 ppm can cause mild irritation of the nose and throat, 250–500 ppm can cause edema, leading to bronchitis or pneumonia, and levels above 1000 ppm can cause death due to asphyxiation from fluid in the lungs. There are often no symptoms at the time of exposure other than transient cough, fatigue or nausea, but over hours inflammation in the lungs causes edema.[31][32]

For skin or eye exposure, the affected area is flushed with saline. For inhalation, oxygen is administered, bronchodilators may be administered, and if there are signs of methemoglobinemia, a condition that arises when nitrogen-based compounds affect the hemoglobin in red blood cells, methylene blue may be administered.[33][34]

It is classified as an extremely hazardous substance in the United States as defined in Section 302 of the U.S. Emergency Planning and Community Right-to-Know Act (42 U.S.C. 11002), and it is subject to strict reporting requirements by facilities which produce, store, or use it in significant quantities.[35]


Possible pathways implicated in long-term nitrogen dioxide exposure. Dotted lines indicate findings only supported by animal studies, while solid lines indicate findings from controlled human exposure studies. Dashed lines indicate speculative links to asthma exacerbation and respiratory tract infections. ELF = epithelial lining fluid.[30]: 4–62 

Exposure to low levels of NO2 over time can cause changes in lung function.[36] Children exposed to NO2 are more likely to be admitted to hospital with asthma.[37] Cooking with a gas stove is correlated with childhood asthma, although as of 2023 a causal link has not been proved.[38]

Environmental effects

Interaction of NO2 and other NOx with water, oxygen and other chemicals in the atmosphere can form acid rain which harms sensitive ecosystems such as lakes and forests.[39] Elevated levels of NO
can also harm vegetation, decreasing growth, and reduce crop yields.[40]

See also


  1. ^ "nitrogen dioxide (CHEBI:33101)". Chemical Entities of Biological Interest (ChEBI). UK: European Bioinformatics Institute. 13 January 2008. Main. Archived from the original on 4 March 2016. Retrieved 4 October 2011.
  2. ^ a b c d Haynes, 4.79.
  3. ^ Mendiara, S. N.; Sagedahl, A.; Perissinotti, L. J. (2001). "An electron paramagnetic resonance study of nitrogen dioxide dissolved in water, carbon tetrachloride and some organic compounds". Applied Magnetic Resonance. 20 (1–2): 275–287. doi:10.1007/BF03162326. S2CID 97875925.
  4. ^ Haynes, 4.134.
  5. ^ Haynes, 5.16.
  6. ^ a b c d e NIOSH Pocket Guide to Chemical Hazards. "Nitrogen dioxide". National Institute for Occupational Safety and Health (NIOSH).
  7. ^ a b "Nitrogen dioxide". Immediately Dangerous to Life or Health Concentrations (IDLH). National Institute for Occupational Safety and Health (NIOSH).
  8. ^ a b Public Domain This article incorporates public domain material from Nitrogen dioxide. United States Environmental Protection Agency. Feb 23, 2016.
  9. ^ "Immediately Dangerous to Life or Health Concentrations (IDLH): Nitrogen dioxide". The National Institute for Occupational Safety and Health (NIOSH). May 1994. Retrieved October 20, 2023.
  10. ^ a b Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. p. 455-7. ISBN 978-0-08-037941-8.
  11. ^ a b Thiemann, Michael; Scheibler, Erich; Wiegand, Karl Wilhelm (2005). "Nitric Acid, Nitrous Acid, and Nitrogen Oxides". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a17_293. ISBN 978-3-527-30673-2.
  12. ^ Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. p. 456. ISBN 978-0-08-037941-8.
  13. ^ a b c Holleman, A. F.; Wiberg, E. (2001) Inorganic Chemistry. Academic Press: San Diego. ISBN 0-12-352651-5.
  14. ^ physical chemistry
  15. ^ Finlayson-Pitts, B. J.; Wingen, L. M.; Sumner, A. L.; Syomin, D.; Ramazan, K. A. (2002-12-16). "The heterogeneous hydrolysis of NO2 in laboratory systems and in outdoor and indoor atmospheres: An integrated mechanism" (PDF). Physical Chemistry Chemical Physics. 5 (2): 223–242. doi:10.1039/B208564J.
  16. ^ Riebsomer, J. L. (1945). "The Reactions of Nitrogen Tetroxide with Organic Compounds". Chemical Reviews. 36 (2): 157–233. doi:10.1021/cr60114a002.
  17. ^ Emil White (1967). "Deamination of Amines. 2-Phenylethyl Benzoate Via the Nitrosoamide Decomposition". Organic Syntheses. 47: 44. doi:10.15227/orgsyn.047.0044.
  18. ^ Smith, Michael B.; March, Jerry (2007), Advanced Organic Chemistry: Reactions, Mechanisms, and Structure (6th ed.), New York: Wiley-Interscience, p. 687, ISBN 978-0-471-72091-1
  19. ^ Subcommittee on Emergency and Continuous Exposure Guidance Levels for Selected Submarine Contaminants; Committee on Toxicology; Board on Environmental Studies and Toxicology; Division on Earth and Life Studies; National Research Council. Chapter 12: Nitrogen Dioxide in Emergency and Continuous Exposure Guidance Levels for Selected Submarine Contaminants. National Academies Press, 2007. ISBN 978-0-309-09225-8
  20. ^ "Mechanism Overview, June 2012" (PDF). Noxilizer, Inc. Archived from the original (PDF) on 12 April 2016. Retrieved 2 July 2013.
  21. ^ Cotton, Simon (21 March 2013) Nitrogen dioxide. RSC Chemistry World.
  22. ^ WHO Air Quality Guidelines – Second Edition. Chapter 7.1 Nitrogen Dioxide.
  23. ^ "Air quality guidelines – global update 2005". WHO. Archived from the original on March 9, 2014. Retrieved 2016-10-19.
  24. ^ US Dept. of Health and Human Services, Public Health Service, Agency for Toxic Substances and Disease Registry, Division of Toxicology. April 2002 ATSDR Nitrous Oxides.
  25. ^ "The Impact of Unvented Gas Heating Appliances on Indoor Nitrogen Dioxide Levels in 'TIGHT' Homes" (PDF). 2013-03-21. Archived from the original (PDF) on 2020-08-05. Retrieved 2018-09-25.
  26. ^ Garrett, Maria H.; Hooper, Martin A.; Hooper, Beverley M.; Abramson, Michael J. (1998-09-01). "Respiratory Symptoms in Children and Indoor Exposure to Nitrogen Dioxide and Gas Stoves" (PDF). American Journal of Respiratory and Critical Care Medicine. 158 (3): 891–895. doi:10.1164/ajrccm.158.3.9701084. PMID 9731022.
  27. ^ Public Domain This article incorporates public domain material from Nitrogen Dioxide Basic Information. United States Environmental Protection Agency. Retrieved February 23, 2016.
  28. ^ Chan-Yeung, M.; Ashley, M. J.; Grzybowski, S. (1978). "Grain dust and the lungs". Canadian Medical Association Journal. 118 (10): 1271–4. PMC 1818652. PMID 348288.
  29. ^ Gurney, J. W.; Unger, J. M.; Dorby, C. A.; Mitby, J. K.; von Essen, S. G. (1991). "Agricultural disorders of the lung". Radiographics. 11 (4): 625–34. doi:10.1148/radiographics.11.4.1887117. PMID 1887117.
  30. ^ a b U.S. EPA. Integrated Science Assessment for Oxides of Nitrogen – Health Criteria (2016 Final Report). U.S. Environmental Protection Agency, Washington, DC, EPA/600/R-15/068, 2016. Federal Register Notice Jan 28, 2016 Free download available at Report page at EPA website.
  31. ^ Toxnet Nitrogen dioxide: Human Health Effects Page accessed March 28, 2016.
  32. ^ CDC NIOSH International Chemical Safety Cards (ICSC): Nitrogen Dioxide Page last reviewed: July 22, 2015; Page last updated: July 1, 2014.
  33. ^ Agency for Toxic Substances and Disease Registry via the CDC Medical Management Guidelines for Nitrogen Oxides Page last reviewed: October 21, 2014; Page last updated: October 21, 2014
  34. ^ University of Kansas Hospital, Poison Control Center Poison Facts: Medium Chemicals: Nitrogen Dioxide Archived 2016-04-11 at the Wayback Machine page accessed March 28, 2016
  35. ^ "40 C.F.R.: Appendix A to Part 355—The List of Extremely Hazardous Substances and Their Threshold Planning Quantities" (PDF) (July 1, 2008 ed.). Government Printing Office. Archived from the original (PDF) on February 25, 2012. Retrieved October 29, 2011.
  36. ^ Int Panis, L (2017). "Short-term air pollution exposure decreases lung function: a repeated measures study in healthy adults". Environmental Health. 16 (1): 60. Bibcode:2017EnvHe..16...60I. doi:10.1186/s12940-017-0271-z. PMC 5471732. PMID 28615020.
  37. ^ Wang, Weiyi; Gulliver, John; Beevers, Sean; Freni Sterrantino, Anna; Davies, Bethan; Atkinson, Richard W.; Fecht, Daniela (2024). "Short-Term Nitrogen Dioxide Exposure and Emergency Hospital Admissions for Asthma in Children: A Case-Crossover Analysis in England". Journal of Asthma and Allergy. 17: 349–359. doi:10.2147/JAA.S448600. ISSN 1178-6965. PMC 11016460. PMID 38623450.
  38. ^ Niranjan, Ajit (2023-11-08). "Gas cookers pump out pollutants linked to childhood asthma, report finds". The Guardian. ISSN 0261-3077. Retrieved 2024-05-07.
  39. ^ US EPA, OAR (2016-07-06). "Basic Information about NO2". US EPA. Retrieved 2020-07-03.
  40. ^ "Nitrogen oxides". Queensland Government. Retrieved 2020-07-03.

Cited sources