Sulphur Dioxide SO2 — What You Need to Know

Sulphur dioxide (SO₂) is a gas that demands respect. Colourless, pungent, and irritating even at low concentrations, SO₂ has long been associated with industrial emissions and environmental pollution. It’s often encountered in smelting operations, chemical manufacturing, and anywhere sulphur-containing fuels are burned.

Yet SO₂ isn’t just a by-product or pollutant. It’s also an essential tool in industrial processing, food preservation, and chemical production. This dual role—as both hazard and asset—makes understanding sulphur dioxide critical for anyone working around it.

In this guide, we’ll break down the main things you need to know about sulphur dioxide: what it is, where it comes from, how it affects human health and the environment, and—most importantly—how to detect and manage it safely in industrial settings.

What is Sulphur Dioxide (SO₂)?

Sulphur dioxide (SO₂) is a colourless, suffocating gas that plays a paradoxical role in modern industry. On one hand, it’s a crucial component in the manufacture of sulphuric acid and used as a preservative in food and drink. On the other, it’s a toxic irritant that can cause serious harm to the respiratory system—even at low concentrations.

Recognised by its sharp, choking odour (often likened to burnt matches), SO₂ is produced both naturally and through human activity. Volcanic eruptions emit it into the atmosphere, but so do industrial operations such as fossil fuel combustion, smelting, and chemical manufacturing.

While it’s less immediately lethal than gases like hydrogen sulphide or carbon monoxide, sulphur dioxide is no less dangerous. It can cause rapid respiratory irritation, trigger asthma attacks, and contribute to environmental issues like acid rain. In confined spaces or poorly ventilated areas, SO₂ can accumulate quickly, especially where sulphur-based fuels or materials are present.

Understanding where SO₂ comes from, how it behaves, and how to detect and manage it, is essential to maintaining safe work environments—particularly in industries like refining, wastewater treatment, pulp and paper, and food processing.

Physical and Chemical Properties

Knowing the physical and chemical traits of sulphur dioxide (SO₂) is essential for understanding how it behaves—and how to control it. Its reactivity, density, and solubility all contribute to its risks in industrial settings, especially where combustion, confined spaces, or chemical processes are involved.

Physical Properties

• Molecular formula: SO₂
• Molecular weight: 64.07 g/mol
• Appearance: Colourless gas
• Odour: Sharp, irritating smell—often compared to burnt matches
• Density: Heavier than air (vapour density ~2.26), which means it tends to settle in low-lying areas
• Solubility: Highly soluble in water, forming sulphurous acid (H₂SO₃)—a weak but corrosive acid

Chemical Properties

• Acidic nature: Reacts readily with water to form acids, contributing to corrosion and environmental damage
• Reactivity: Reacts with oxidising agents, water, and many organic compounds; also participates in atmospheric reactions leading to sulphuric acid aerosols
• Non-flammable: SO₂ itself doesn’t burn, but it can act as an oxidiser under certain conditions
• Combustion by-product: Often formed when sulphur-containing materials or gases (like H₂S) are burned
• Corrosiveness: While less aggressive than H₂S in forming metal sulphides, SO₂ and its aqueous forms can corrode metals, plastics, and rubber over time

These properties make SO₂ both a practical industrial chemical and a potential workplace hazard. Its tendency to accumulate, irritate, and corrode—especially when combined with water vapour—means that careful monitoring and ventilation are non-negotiable in areas where it’s present.

Sources of Sulphur Dioxide

Sulphur dioxide (SO₂) is both a naturally occurring gas and a by-product of human activity—particularly wherever sulphur-containing materials are burned or processed. Understanding its sources is critical for identifying risk zones and designing effective monitoring systems.

Natural Sources

• Volcanic activity: One of the largest natural emitters of SO₂, volcanic eruptions can release vast amounts of the gas into the atmosphere, contributing to short-term air pollution and long-term climatic effects.
• Geothermal vents and hot springs: Areas with geothermal activity can emit SO₂ along with other sulphur gases, often unnoticed by those nearby.
• Biological decay (minor): In rare cases, the anaerobic breakdown of organic matter may produce small quantities of SO₂, although this is more commonly associated with hydrogen sulphide.

Industrial Sources

• Fossil fuel combustion: The largest man-made source of SO₂. Burning coal, oil, or diesel with high sulphur content—particularly in power plants and heavy industry—produces significant SO₂ emissions.
• Metal smelting and refining: Processing ores that contain sulphur (e.g., copper, nickel, zinc) releases SO₂ during roasting and reduction stages.
• Sulphuric acid production: SO₂ is both a raw material and an intermediate in the production of sulphuric acid through the Contact Process.
• Pulp and paper mills: Some sulphite pulping methods use SO₂ or produce it as a by-product during wood breakdown.
• Food and beverage preservation: SO₂ is intentionally used in small amounts as a preservative and antimicrobial in wine, dried fruits, and some processed foods—but these quantities are controlled and regulated.
• Waste treatment and refrigeration (legacy systems): Older systems and certain chemical treatment processes may release SO₂ under specific conditions, especially during incineration or thermal treatment.

With such a broad range of potential sources—from industrial giants to niche applications—it’s vital that workplaces assess not just obvious emission points, but also secondary processes and older infrastructure that may contribute to background SO₂ levels.

Health Hazards of SO₂ Exposure

Sulphur dioxide (SO₂) is a potent respiratory irritant. While not as acutely toxic as some other industrial gases, its effects can be swift, severe, and long-lasting—especially for individuals with pre-existing respiratory conditions. Even low-level exposure can cause discomfort, while higher concentrations pose serious health risks.

Acute Effects

• Respiratory irritation: Inhalation of SO₂ can cause immediate coughing, throat tightness, and a burning sensation in the nose and chest.
• Bronchoconstriction: SO₂ can trigger airway narrowing, especially in people with asthma, often within minutes of exposure.
• Eye and mucous membrane irritation: Exposure can lead to red, watery eyes and a sore throat, even at relatively low concentrations.
• Shortness of breath and wheezing: These symptoms may occur rapidly, particularly during physical activity in contaminated environments.
• Nausea and dizziness: Common in enclosed or poorly ventilated areas with elevated concentrations.

Chronic Exposure Risks

Prolonged or repeated exposure to sulphur dioxide—especially at low-to-moderate levels—can lead to:

• Asthma aggravation: Regular exposure may increase the frequency or severity of asthma attacks.
• Reduced lung function: Long-term inhalation can contribute to chronic bronchitis or other obstructive respiratory conditions.
• Heightened sensitivity: Once sensitised, individuals may react to even minimal exposures with strong symptoms.

Concentration and Effects

High-Risk Groups

Certain individuals are significantly more vulnerable to SO₂ exposure, including:

• Asthmatics and those with chronic respiratory conditions
• Children and the elderly
• Workers in smelting, refining, chemical manufacturing, and pulp industries
• Anyone exposed in confined or poorly ventilated spaces

Early recognition of symptoms, prompt use of detection equipment, and access to clean air are critical to managing SO₂ exposure. Unlike some gases, SO₂ does have a strong odour—but relying on smell alone is unreliable, especially in fast-moving work environments where adaptation or other odours can mask its presence.

Industrial and Scientific Applications

Despite its reputation as an air pollutant, sulphur dioxide (SO₂) plays a critical role in multiple industrial and scientific settings. When handled properly, it becomes a versatile tool—used in everything from chemical manufacturing to food preservation. But because of its toxic and corrosive nature, every application must be managed with robust safety controls.

Industrial Uses

• Sulphuric acid production
  The most common industrial use of SO₂ is as a feedstock in the production of sulphuric acid via the Contact Process. Sulphuric acid is essential for fertilisers, batteries, and chemical manufacturing.
• Metal refining and smelting
  During the extraction and refining of metals like copper, zinc, and lead, sulphur-containing ores release SO₂ gas. It may be captured and recycled for sulphuric acid production or released (and scrubbed) through exhaust systems.
• Bleaching agent in pulp and paper
  SO₂ is used in sulphite pulping to break down lignin in wood chips, helping to produce lighter-coloured paper products.
• Refrigeration (historical use)
  In the early 20th century, SO₂ was used as a refrigerant. While now obsolete due to toxicity, legacy systems may still pose risks during demolition or maintenance.
• Preservative in food and beverages
  SO₂ is approved in small amounts for use as a preservative and antimicrobial agent—particularly in wine, dried fruits, and some canned products. It helps prevent browning and spoilage, though regulations strictly limit its concentration.

Scientific and Laboratory Uses

• Fumigation and disinfection
  In controlled environments, SO₂ can be used for fumigating shipping containers, food storage areas, and historical archives (though this is less common today due to safety concerns).
• Chemical synthesis
  SO₂ is used as a reagent in organic and inorganic chemistry—especially in the creation of sulphonates and sulphites.
• Analytical applications
  SO₂ may be involved in titration and other laboratory methods where reducing agents are required.

Although these uses are well established, they come with strict regulatory and safety requirements. Any process involving SO₂ must include real-time monitoring, proper ventilation, and fully trained personnel equipped with appropriate PPE.