Sulfur Dioxide

Classification

Gas under pressure (liquefied gas); Acute toxicity (inhalation), Category 3; Skin corrosion, Category 1B; Serious eye damage, Category 1

Hazard statements

• H280 Contains gas under pressure; may explode if heated.
• H314 Causes severe skin burns and eye damage.
• H331 Toxic if inhaled.

Key precautionary statements

• P260 Do not breathe gas.
• P271 Use only outdoors or in a well-ventilated area.
• P280 Wear protective gloves, protective clothing, eye protection and face protection.
• P304+P340+P310 IF INHALED: Remove person to fresh air and keep comfortable for breathing. Immediately call a POISON CENTER or doctor.
• P305+P351+P338+P310 IF IN EYES: Rinse cautiously with water for several minutes. Remove contact lenses, if present and easy to do. Continue rinsing. Immediately call a POISON CENTER or doctor.
• P405 Store locked up.
• P410+P403 Protect from sunlight. Store in a well-ventilated place.

The federal OSHA PEL is 5 ppm (13 mg/m³) as an 8-hour TWA, from Table Z-1. NIOSH recommends a much lower 2 ppm TWA with a 5 ppm STEL, and Cal/OSHA has adopted 2 ppm TWA and 5 ppm STEL. ACGIH set a TLV-STEL of 0.25 ppm in 2008 with no separate TWA, reflecting how strongly even brief peaks affect people with asthma. The IDLH is 100 ppm. The odor is noticeable at low levels, but people get used to it, so rely on gas detection rather than smell. State OSHA plans may set different limits.

• Appearance: Colorless gas; shipped as a liquefied compressed gas
• Odor: Sharp, irritating, choking odor (like a struck match)
• Molecular weight: 64.1
• Boiling point: 14°F (-10°C)
• Freezing point: -104°F (-76°C)
• Flash point: Not applicable (nonflammable gas)
• Explosive limits: Not applicable
• Vapor pressure: 3.2 atm
• Relative gas density: 2.26 (more than twice as heavy as air)
• Water solubility: 10%; reacts with water to form sulfurous acid

Inhalation

Irritates the nose, throat and lungs, causing runny nose, coughing, choking and tightness in the chest. It triggers airway narrowing (bronchoconstriction), and people with asthma can react to very low levels. High exposures can cause fluid in the lungs (pulmonary edema), which can be delayed, and death.

Skin

The gas forms acid on moist skin and causes irritation and burns. Contact with the liquid causes frostbite.

Eyes

Causes serious eye damage. The gas irritates and burns; liquid splashes freeze and burn the eye and can cause permanent damage.

Ingestion

Not a relevant route for a gas.

Target organs

Eyes, skin, respiratory system

Long-term effects

Repeated exposure can cause chronic bronchitis, reduced lung function and a lasting asthma-like condition. IARC places sulfur dioxide in Group 3 (not classifiable as to carcinogenicity in humans).

Gloves

One supplier SDS lists butyl rubber for full contact (480-minute breakthrough at 0.3 mm) and chloroprene for splash contact. Use insulated gloves over chemical gloves where liquid could contact the hands.

Eyes and face

Non-vented chemical goggles and a face shield when connecting or changing cylinders. Provide an eyewash and a frostbite wash.

Respiratory

NIOSH lists chemical cartridge respirators with sulfur dioxide cartridges up to 20 ppm, powered or full-facepiece respirators up to 100 ppm, and a pressure-demand SCBA for emergencies or unknown concentrations. Use a full facepiece where possible because the gas also attacks the eyes, as part of a written respiratory protection program.

Clothing

Chemical-resistant clothing for cylinder work. Responders to leaks need gas-tight chemical protective suits with SCBA. Remove clothing that is wet with liquid.

Store cylinders upright and secured, with valve caps on, in a cool, dry, well-ventilated, locked area away from heat and direct sunlight. Keep cylinder temperature below 125°F (52°C).

Use only in closed systems or with good local exhaust ventilation, with fixed gas detection where cylinders are connected. Moist sulfur dioxide is very corrosive because it forms sulfurous acid, and in air it can oxidize further toward sulfuric acid, so keep systems dry.

Keep away from

• Water and moisture
• Ammonia
• Powdered alkali metals such as sodium and potassium
• Zinc, aluminum, brass and copper
• Strong oxidizers and strong bases

• Evacuate the area and isolate it in all directions; call emergency responders.
• Do not enter without a positive-pressure SCBA and gas-tight protective clothing.
• Stop the leak if it can be done safely and ventilate the area.
• Use water spray or fog to knock down and absorb the gas, but do not spray water directly on a leaking cylinder or onto pooled liquid.
• Keep people upwind and out of low areas, because the gas is heavier than air.
• Follow ERG Guide 125.

Suitable extinguishing media

• Sulfur dioxide does not burn; use media suited to the surrounding fire
• Water spray to cool fire-exposed cylinders from a distance
• Do not get water inside cylinders or on leaking valves

Special hazards

Cylinders can rupture when heated, releasing large amounts of toxic gas. Firefighters must wear positive-pressure SCBA and full protective gear. Sulfur dioxide is also formed when sulfur-containing fuels or materials burn, and hot work on sulfur-bearing materials can release it.

• OSHA Hazard Communication (29 CFR 1910.1200): Hazardous chemical. Labels and safety data sheets are required.
• OSHA Air Contaminants (29 CFR 1910.1000, Table Z-1): PEL 5 ppm (13 mg/m³) TWA.
• OSHA Process Safety Management (29 CFR 1910.119): Liquid sulfur dioxide is a highly hazardous chemical with a threshold quantity of 1,000 lb.
• EPCRA Section 302 (40 CFR 355): Extremely hazardous substance; threshold planning quantity 500 lb; Section 304 reportable quantity 500 lb.
• CERCLA (40 CFR 302.4): Not listed as a CERCLA hazardous substance.
• EPCRA Section 313 / TRI: Not listed.
• Clean Air Act 112(r) (40 CFR 68.130): Sulfur dioxide (anhydrous) is a regulated toxic substance; threshold quantity 5,000 lb. It is also a criteria air pollutant under the national ambient air quality standards.
• DOT (49 CFR 172.101): UN1079 Sulfur dioxide, Division 2.3 with subsidiary hazard 8; poison inhalation hazard, Hazard Zone C.

• NIOSH Pocket Guide: Sulfur dioxide
• OSHA Occupational Chemical Database: Sulfur Dioxide
• NOAA CAMEO Chemicals: Sulfur Dioxide
• Sigma-Aldrich SDS: Sulfur dioxide