Sulfur candles are blocks or sticks of solid sulfur designed to be lit and burned in enclosed spaces, releasing sulfur dioxide gas to kill fungi, mites, and insects. They’ve been used for decades in greenhouses, wine barrels, and storage areas, but their legal status has shifted significantly. No sulfur products are currently registered by the EPA for use as fumigants in enclosed spaces, making this method effectively illegal for pest control in the United States.
How Sulfur Candles Work
When a sulfur candle burns, the solid sulfur reacts with oxygen in the air to produce sulfur dioxide, a colorless gas with a sharp, pungent smell. This gas is toxic to a wide range of organisms. It kills fungal spores on contact, suffocates mites, and drives out or kills insects in enclosed areas. The gas penetrates cracks, soil surfaces, and porous materials that sprays often miss, which is why growers historically favored it for greenhouse use.
Sulfur dioxide also reacts with moisture in the air to form sulfurous acid. This is part of what makes it effective against pathogens, but it’s also what makes it destructive. That acid damages plant leaves, corrodes metal tools and greenhouse frames, and can etch glass over time. Any plants left in a treated space during burning will likely suffer visible leaf burn.
Traditional Uses
Sulfur candles were most commonly associated with greenhouse growing, where powdery mildew is a persistent problem on crops like tomatoes, kale, and lettuce. Growers would burn sulfur overnight in empty or evacuated greenhouses to coat surfaces with a thin residue of sulfur and kill mildew spores before they could establish. The candles were also used against spider mites and rust diseases.
Outside of agriculture, sulfur candles have been used to sanitize wine and beer barrels, fumigate root cellars, and treat crawl spaces or basements for mold. Beekeepers sometimes used them to sterilize hive equipment. In all of these applications, the principle is the same: fill a sealed space with sulfur dioxide, let it sit, then ventilate thoroughly before anyone enters.
Why They’re No Longer Legal
The California Department of Pesticide Regulation issued a clarification that burning sulfur to control pests in enclosed spaces is a pesticidal use requiring registration by both the EPA and state regulators. No sulfur products are currently registered for this application method. There are roughly 80 registered pesticide products containing sulfur in the U.S., 34 of which contain at least 80% sulfur, but none are approved for burning as a fumigant.
Cornell University’s vegetable program reached the same conclusion: sulfur burners and evaporators cannot be legally used for managing diseases in commercial crops in greenhouses and high tunnels. Using a registered sulfur product in a way that contradicts its label (and most sulfur product labels explicitly say “keep away from heat, sparks, or flames”) violates federal and state pesticide law.
This applies to commercial growers. Home gardeners technically fall under the same federal rules, though enforcement is directed at commercial operations. Regardless of legality, the safety risks are real for anyone.
Health Risks of Sulfur Dioxide Exposure
Sulfur dioxide is genuinely dangerous to breathe. OSHA sets the workplace exposure limit at 5 parts per million over an eight-hour shift. The concentration considered immediately dangerous to life or health is 100 ppm. A burning sulfur candle in a small, sealed room can easily exceed both of those thresholds within minutes.
At low concentrations, sulfur dioxide irritates the eyes, nose, and throat. At higher levels, it causes coughing, chest tightness, and difficulty breathing. People with asthma are especially vulnerable and can experience severe bronchospasm at concentrations that might only mildly irritate someone else. Prolonged exposure at high concentrations can cause pulmonary edema, a life-threatening buildup of fluid in the lungs.
The American Industrial Hygiene Association sets its lowest emergency threshold at just 0.3 ppm, the level at which nearly all people would notice irritation. That gives you a sense of how little sulfur dioxide it takes to cause problems. Anyone using sulfur candles without proper respiratory protection and a clear ventilation plan is taking a serious risk.
Damage to Property and Plants
The sulfurous acid that forms when the gas meets humidity doesn’t just harm lungs. It corrodes copper, iron, and steel. Greenhouse hardware, electrical components, and metal shelving in treated spaces can deteriorate rapidly with repeated exposure. Galvanized surfaces hold up somewhat better, but no metal is truly immune.
Plants exposed during treatment develop characteristic bleached or burned spots on their leaves. Even residual gas lingering after inadequate ventilation can cause damage to sensitive crops brought back into the space too soon. Growers who used sulfur candles historically had to remove all plants, seal the space, burn the candle, and then ventilate for hours (sometimes a full day) before reintroducing any living plants.
Alternatives for Fungus and Mite Control
If you’re dealing with powdery mildew in a greenhouse or grow room, several EPA-registered sulfur products are available as dusts, wettable powders, and liquid sprays that don’t require burning. These are applied directly to plant surfaces at labeled rates and are far easier to use safely. They work through direct contact with fungal spores rather than gas fumigation.
For mites, horticultural oils and insecticidal soaps are widely available and effective when applied thoroughly to leaf undersides. Biological controls like predatory mites (particularly Phytoseiulus persimilis for spider mites) offer longer-term suppression without chemical residue. Improving air circulation and reducing humidity in enclosed growing spaces also makes conditions less favorable for both powdery mildew and mite populations.
For non-agricultural spaces like basements or crawl spaces with mold issues, addressing the underlying moisture problem is more effective than any fumigant. Dehumidifiers, improved drainage, and vapor barriers solve the root cause. Surface mold on hard materials can be treated with hydrogen peroxide or commercial mold removers that don’t carry the risks of filling a space with toxic gas.