Identifiers for search
| Name | Sulfur dioxide |
| Formula | SO₂ |
| IUPAC name | sulfur dioxide |
| CAS number | 7446-09-5 |
| EC number | 231-195-2 |
| UN number | 1079 — Sulphur dioxide |
| Transport class (ADR/RID) | 2.3 (Toxic gases) |
| PubChem CID | 1119 |
| Synonyms | Sulfur oxide, Sulfurous acid anhydride, Sulfurous oxide |
Physical & chemical properties
| Molar mass | 64.06 g/mol |
| Boiling point (1 atm) | -10 °C |
| Melting point | -75.5 °C |
| Critical temperature | 157.5 °C |
| Critical pressure | 78.8 bar |
| Vapour pressure | 3.3 bar at 20 °C |
| Gas density | 2.9285 kg/m³ (0 °C, 1013 mbar) |
| Relative density (air = 1) | 2.26 |
| Solubility in water | 228,3 g/l at 0 °C |
| Appearance | gas liquefied under pressure, colourless, pungent odour |
| Odour | pungent odour |
| 1 ppm at 20 °C | 2.66 mg/m³ |
Behaviour with temperature and pressure
Boiling point -10 °C at 1 atm: a liquefied gas under its own vapour pressure in cylinders. Critical point 157.5 °C / 78.8 bar: below this temperature it can be stored as a liquid; cylinder pressure then equals the vapour pressure. Vapour pressure ≈ 3.3 bar at 20 °C — this is the pressure inside a cylinder or a switchgear compartment at that temperature, and it rises steeply with temperature (Clausius–Clapeyron). Relative density 2.26 (air = 1): heavier than air — accumulates in pits, cable basements and at floor level; place sensors low. Conversion at 20 °C, 1013 mbar: 1 ppm = 2.66 mg/m³; NDIR reads partial pressure, so readings scale with absolute pressure (value = reading × P/1013) and must be pressure-compensated.
// Non-combustible gas., Soluble in water, hydrolyses., Forms sulfurous acid in water., Binds moisture from air and forms aerosoles!, Aqueous solution reacts strongly acidic., Gas is heavier than air.
Hazards & safety
NFPA 704
health · flammability · instability · special
GHS / CLP pictograms
| Hazard class | toxic (corrosive, respiratory irritant) |
| GHS signal word | Danger |
| CLP classification | Gases under pressure, liquefied gas; H280 Acute toxicity, Category 3, inhalation; H331 Skin corrosion, Category 1B; H314 Serious eye damage, Category 1; H318 Specific Target Organ Toxicity (single exposure), Category 1; H370 |
| Hazard statements | H280: Contains gas under pressure; may explode if heated. H314: Causes severe skin burns and eye damage. H331: Toxic if inhaled. H370: Causes damage to organs. EUH071: Corrosive to the respiratory tract. |
| Lower / upper explosion limit | non-flammable |
| Auto-ignition temperature | — |
| Seveso III named substance | yes |
Exposure limits
| EU indicative OELV (8 h TWA) | 0.5 ppm · 1.3 mg/m³ — Directive 2017/164/EU |
| EU IOELV short-term (15 min) | 1 ppm · 2.7 mg/m³ |
| Germany AGW (TRGS 900) | 0.5 ppm · 1.3 mg/m³ |
| Netherlands | wettelijke grenswaarde per Arboregeling bijlage XIII — SER grenswaarden database |
| NIOSH REL (USA) | TWA 2 ppm (5 mg/m3) ST 5 ppm (13 mg/m3) |
| OSHA PEL (USA) | TWA 5 ppm (13 mg/m3) See Appendix G |
| IDLH | 100 ppm |
// Limits change: always confirm the current national value before setting an alarm. Values above were read from the EU directives / GESTIS / NIOSH pages listed in Sources.
Effect on people
Strong respiratory irritant: forms sulfurous acid on wet mucosa. Coughing and bronchoconstriction from ~1–5 ppm (asthmatics far lower), pulmonary oedema from ~100 ppm (IDLH), rapidly fatal above ~400 ppm. EU IOELV 0.5 ppm.
GESTIS main toxic effects: Acute:
Irritation to the eyes and airways,
chemical burns to the eyes and skin through liquified SO2
Chronic:
Irritation to the respiratory tract
Recommended monitoring
- Electrochemical for ppm workplace detection; NDIR/UV for CEMS at mg/m³ under IED / EN 14181 with QAL procedures; heated sample lines are mandatory (SO₂ dissolves in condensate).
- Alarm at 0.5 ppm (IOELV) — very low: EC cells with H₂S filters or UV fluorescence are used; NDIR suits stack ranges (0–500 mg/m³ and up).
Climate & atmosphere
| GWP₁₀₀ — IPCC AR6 | — |
| GWP₁₀₀ — IPCC AR4 | — |
| GWP used in Reg. (EU) 2024/573 | not listed |
| Atmospheric lifetime | — |
| Ozone depletion potential | 0 |
| Main IR absorption band (NDIR) | 7.3 µm (ν₃, 1362 cm⁻¹); 8.7 µm (ν₁, 1151 cm⁻¹); 4.0 µm overtone |
Regulation windows
- IED 2024/1785 / LCP BREF — SO₂ ELVs, CEMS with EN 14181
- Workplace: EU IOELV 0.5 ppm / STEL 1 ppm (Dir. 2017/164/EU)
- Ambient air: Directive 2024/2881 limit values
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Cite this page
Tremon Solutions (2026). Sulfur dioxide (SO₂) — properties, exposure limits, NDIR monitoring. Tremon gas reference. https://www.tremon.nl/gases/so2 (accessed 2026-09-17).
BibTeX · embed card · QR
@misc{tremon_so2_2026,
author = {Tremon Solutions},
title = {Sulfur dioxide (O2S) — properties, exposure limits, NDIR monitoring},
howpublished = {Tremon gas reference, \url{https://www.tremon.nl/gases/so2}},
year = {2026},
note = {accessed 2026-09-17}
}<a href="https://www.tremon.nl/gases/so2"><img src="https://www.tremon.nl/assets/og/so2.png" alt="Sulfur dioxide — Tremon gas reference" width="600"></a>
Text and tables on this page: CC BY 4.0 — copy them into your report, SOP or slide with the link above. Data were read from the primary sources listed below; check the current national limit before setting an alarm.
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Other gases
https://www.tremon.nl/gases/so2 · Tremon Solutions, Eindhoven · [email protected] · printed from the Tremon gas reference, data as of 2026-09-17 · CC BY 4.0Sources
- https://gestis-database.dguv.de/data?name=001020
- https://www.cdc.gov/niosh/npg/npgd0575.html
- https://pubchem.ncbi.nlm.nih.gov/compound/1119
- IPCC AR6 WG1 Ch.7 Supplementary Material, Table 7.SM.7 (https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_Chapter07_SM.pdf)
- HITRAN/NIST IR band positions (approximate centre of the strongest band used for NDIR; https://hitran.org, https://webbook.nist.gov)
// Data compiled 2026-09-17 from the listed primary sources; null values were left blank rather than estimated.