Most polymers are flammable, and many burn readily. Plastics like polyethylene, polypropylene, and polyurethane foam will ignite, sustain a flame, and release toxic gases. However, “polymer” covers an enormous range of materials, and some are engineered to resist fire or even self-extinguish. The answer depends entirely on which polymer you’re talking about and whether it contains flame-retardant additives.
Why Most Polymers Burn
Polymers are long chains of carbon-based molecules, and carbon burns. When heat reaches a polymer surface, the first stage of combustion is pyrolysis: the material breaks down into smaller, volatile gases. Those gases mix with oxygen in the air and ignite, creating a flame that radiates more heat back onto the surface, breaking down more material. This self-feeding cycle is why plastic fires can escalate quickly and are difficult to control once established.
Thermoplastics (the kind used in packaging, furniture, and consumer goods) add another problem. They soften and melt as they burn, and the movement of burning molten material can spread fire to surfaces below or nearby. The National Institute of Standards and Technology has flagged this dripping behavior as a distinct challenge for fire safety. Thermoset polymers, by contrast, tend to char in place rather than melt, which limits fire spread but doesn’t prevent burning.
How Flammability Is Measured
One of the most common ways to compare polymer flammability is the Limiting Oxygen Index, or LOI. This is the minimum percentage of oxygen needed to keep the material burning. Normal air contains about 21% oxygen, so any polymer with an LOI below 21% will sustain a flame in open air without an external heat source.
- Polypropylene: LOI of 18%, meaning it burns easily in normal air
- Polybutylene terephthalate (PBT): LOI of 20%, still flammable in air
- Nylon 6,6: LOI of 24%, marginally self-extinguishing in air
- Polyaramid (Kevlar-type fiber): LOI of 38%, highly resistant to flame
The higher the LOI, the harder it is to ignite. Materials above 21% will generally stop burning once an external flame is removed, though intense heat sources can still push them past their threshold. Polypropylene and polyethylene, two of the most common plastics on the planet, sit well below that line.
Manufacturers also use the UL 94 rating system. A sample is held vertically and exposed to a flame, then rated based on how quickly it self-extinguishes, whether it burns to the top of the sample, and whether it drips molten material that ignites cotton placed below. The top rating, V-0, requires the flame to go out almost immediately with no flaming drips. Many commodity plastics cannot achieve this rating without additives.
Polymers That Resist Fire
A small category of high-performance polymers is inherently flame resistant without needing chemical additives. Polyetherimide (PEI) and polyetherketoneketone (PEKK) are used in aerospace interiors precisely because they self-extinguish and produce minimal flame spread. In 60-second vertical burn tests used for aviation certification, PEI samples burned only 10 to 15 millimeters before self-extinguishing. PEKK burned further, around 62 to 73 millimeters, but both materials passed the flammability requirements set by aviation regulators for cabin interiors.
PTFE (Teflon) is another example, with an exceptionally high LOI that makes it nearly impossible to ignite under normal conditions. These materials cost significantly more than common plastics, so they’re reserved for applications where fire resistance is critical.
What Happens When Polymers Burn
The gases released by burning polymers are often more dangerous than the flames themselves. All burning plastics produce carbon monoxide, which starves cells of oxygen and causes loss of consciousness. Many also release carbon dioxide, aldehydes, and a mix of hydrocarbon fragments.
Some polymers release especially hazardous gases tied to their chemical structure. PVC (polyvinyl chloride) undergoes a reaction called dehydrochlorination at temperatures between 200 and 300°C, releasing hydrogen chloride gas. This is a severe irritant to the eyes, throat, and lungs. Polyurethane foams, widely used in mattresses, upholstered furniture, and insulation, can release hydrogen cyanide in addition to carbon monoxide. Both are classified as asphyxiant gases that prevent cells from using oxygen, and their yield increases substantially when a fire is oxygen-starved, such as in an enclosed room with limited ventilation.
Irritant gases from polymer fires cause immediate effects on the eyes and upper airways, while deeper lung damage can develop over hours. In real-world house fires, the toxic smoke from burning plastics and foams is the primary cause of death, not the flames.
How Flame Retardants Work
Since most commercial polymers burn easily on their own, manufacturers add flame-retardant chemicals to slow ignition and reduce flame spread. One common approach uses intumescent additives, which swell when heated and form a thick carbonaceous char layer on the surface. This char acts as a physical barrier, insulating the underlying material from heat and cutting off the supply of combustible gases that feed the flame. Products with multiple filler compounds tend to produce a more uniform, intact char layer, which improves protection.
Other flame retardants work in the gas phase, releasing chemicals that interrupt the combustion reactions happening in the flame itself. Halogenated retardants (containing bromine or chlorine) have been widely used for decades because they’re effective and inexpensive. However, many of these compounds persist in the environment and accumulate in the body.
Tightening Rules on Flame Retardant Chemicals
The European Union has been steadily restricting brominated flame retardants. In October 2025, the European Commission adopted new rules that slash the allowable concentration of polybrominated diphenyl ethers (PBDEs) in consumer products from 500 milligrams per kilogram down to just 10 mg/kg. Recycled materials get a transitional timeline, dropping to 350 mg/kg by late 2025 and 200 mg/kg by the end of 2027. Toys and childcare products must meet the strictest 10 mg/kg limit by mid-2027.
These regulations are pushing manufacturers toward halogen-free alternatives, including phosphorus-based and mineral-based retardants. For consumers, this means newer products may use different fire-safety chemistry, but the underlying flammability of the base polymer hasn’t changed. The additives are simply evolving to balance fire safety with environmental and health concerns.
Practical Takeaways
If you’re evaluating a specific plastic product for fire risk, look for its UL 94 rating or ask the manufacturer for its LOI value. Common household plastics like polyethylene, polypropylene, and polystyrene are highly flammable without additives. Polyurethane foam ignites readily and produces particularly toxic smoke. PVC is slower to ignite but releases corrosive hydrogen chloride gas when it does burn.
For applications where fire resistance matters, such as building insulation, electrical enclosures, or transportation interiors, engineering-grade polymers like PEI and PEKK offer inherent flame resistance. For everything else, flame-retardant additives are the standard solution, though the specific chemicals used are changing rapidly under new environmental regulations.