Phosphine is a colorless, highly toxic, and flammable gas made of one phosphorus atom bonded to three hydrogen atoms (PH₃). It occurs naturally in trace amounts in swamps and marshes, but its primary significance today is as a widely used fumigant in agriculture and a dopant in semiconductor manufacturing. Workplace exposure limits are set at just 0.3 parts per million, reflecting how dangerous even small concentrations can be.
Chemical Properties
Phosphine has a molecular weight of about 34 g/mol and boils at a frigid -126°F (-88°C), meaning it exists as a gas under any conditions you’d encounter in daily life. It dissolves poorly in water and is slightly soluble in alcohol and ether.
The gas is extremely flammable. Its explosive range in air starts at just 1.8%, and gas-air mixtures can ignite from heat, sparks, or open flames. Pure phosphine is actually odorless, but in practice it almost always contains trace impurities, particularly a related compound called diphosphine, that give it a distinctive garlic or rotting fish smell. The odor becomes detectable at roughly 0.14 ppm, though relying on smell alone is unreliable for safety purposes. Perhaps most alarming, phosphine’s autoignition temperature is below 32°F (0°C), which means impure samples can ignite spontaneously on contact with air at room temperature.
Where Phosphine Comes From
Most phosphine in the atmosphere is biological in origin. Microorganisms in oxygen-free environments, particularly marshes, swamps, and lake sediments, produce it as part of the phosphorus cycle. This is why phosphine has historically been linked to “swamp gas” and the eerie, flickering lights sometimes reported over wetlands. The gas forms when bacteria break down phosphorus-containing organic matter under strictly anaerobic (oxygen-free) conditions.
Human activity adds substantially to phosphine levels. Countries with heavy agricultural activity tend to have higher ambient phosphine concentrations, largely from the use of metal phosphide pesticides. Urban and industrial areas contribute as well.
Industrial and Agricultural Uses
Phosphine’s most common application is as a fumigant. Aluminum phosphide tablets are placed into grain storage containers, ship cargo holds, or warehouses, where they react with moisture in the air to release phosphine gas. The gas penetrates bulk materials like grain, rice, and wood, killing insects and rodents by disrupting their cellular respiration and metabolism. Shipping containers carrying agricultural and forestry products are routinely fumigated this way to prevent infestations during transit.
This widespread use has also led to serious accidental poisonings. In 1980, two children and 29 crew members aboard a grain freighter fell ill after four days of phosphine exposure. More recently, in 2024, a family in the Dominican Republic was poisoned after their apartment building was fumigated for a woodworm infestation without notifying residents.
In the semiconductor industry, phosphine serves as a source of phosphorus for doping silicon wafers. During chip manufacturing, controlled amounts of phosphine gas introduce phosphorus atoms into silicon crystals, creating what engineers call n-type semiconductors. These modified silicon layers are essential building blocks in computer chips, solar cells, and other electronic devices. Phosphine also serves as a precursor for flame retardants and chemicals used in metal extraction.
How Phosphine Harms the Body
Phosphine is toxic because it shuts down the way your cells produce energy. Inside every cell, structures called mitochondria use oxygen to generate the fuel molecule ATP through a chain of chemical reactions. Phosphine blocks a critical step near the end of that chain, specifically the enzyme complex responsible for the final transfer of electrons to oxygen. When that step fails, the entire energy production system stalls.
The consequences cascade quickly. With the normal energy pathway disabled, cells switch to a far less efficient backup process that produces lactic acid as a byproduct. Meanwhile, the disrupted mitochondria begin leaking highly reactive molecules (free radicals) that damage proteins, fats, and DNA. This oxidative damage can trigger programmed cell death across multiple organs. Animal studies show that acute exposure causes a measurable spike in the enzyme lactate dehydrogenase in the brain, a clear marker that the organ has shifted from its normal oxygen-based metabolism to emergency anaerobic mode.
There is no antidote for phosphine poisoning. Treatment is entirely supportive: removing the person from exposure, providing oxygen, and monitoring heart and lung function.
Symptoms of Exposure
Inhaling phosphine primarily damages the lungs, heart, and liver. At low concentrations, early symptoms include chest tightness, shortness of breath, nausea, and fatigue. Higher or prolonged exposure can cause fluid buildup in the lungs, dangerously low blood pressure, irregular heart rhythms, seizures, and respiratory failure. Because the gas disrupts energy production at the cellular level, virtually any organ can be affected in severe cases. Symptoms can develop rapidly after a significant inhalation exposure, and the damage to lung tissue can worsen over the hours following exposure even after the person is removed from the contaminated area.
Workplace Safety Limits
Both OSHA and NIOSH set the permissible workplace exposure at 0.3 ppm as an 8-hour average. NIOSH also sets a short-term ceiling of 1 ppm, meaning workers should never exceed that concentration even briefly during a shift. The “immediately dangerous to life or health” threshold is 50 ppm, the concentration at which a 30-minute exposure could cause death or irreversible harm.
To put those numbers in context, you can smell phosphine at around 0.14 ppm, which is below the workplace limit. But odor perception varies widely between individuals, and at higher concentrations phosphine can actually dull your sense of smell, making it less noticeable precisely when it’s most dangerous. Workplaces that handle phosphine rely on electronic gas detectors rather than human senses.
Phosphine on Venus
In 2020, a team of astronomers reported detecting phosphine in the cloud decks of Venus at roughly 20 parts per billion, using observations from the James Clerk Maxwell Telescope and the ALMA Observatory. The finding drew enormous attention because no known non-biological process could account for phosphine in Venus’s atmosphere, where phosphorus should exist only in oxidized forms. The researchers exhaustively ruled out lightning, volcanic activity, meteorite delivery, and known chemical pathways.
The claim remains contested. Errors were later identified in the processing of the ALMA data, and the journal Nature Astronomy issued a caution against relying on the original quantitative measurements. Whether Venus truly harbors phosphine, and whether it could point to microbial life in the planet’s clouds, is still an open question. The detection reinvigorated interest in Venus missions, with several space agencies now planning probes that could settle the debate.