Spray foam that has fully and properly cured is largely stable, but it is not completely inert. The reactive chemicals that make spray foam dangerous during application, primarily isocyanates, do polymerize and become bound within the hardened material. However, other compounds in the foam, particularly flame retardants, continue to release into indoor air at measurable levels for well over 18 months. The real answer depends on whether the foam cured correctly and what chemicals you’re concerned about.
What’s in Spray Foam Once It Hardens
Spray polyurethane foam starts as two liquid components that react on contact. Side A contains isocyanates, the chemicals responsible for most acute health effects. Side B contains a polyol along with catalysts, flame retardants, blowing agents, and surfactants. When the two sides mix at the correct ratio and temperature, the isocyanates react with the polyol and lock into a solid polymer structure.
Once that reaction is complete, the isocyanates are chemically bound and no longer free to enter the air in significant quantities. Amine catalysts, which can cause a fishy smell during installation, also tend to dissipate. In NIST testing of foam aged 1.5 years, amine catalysts were no longer detectable. But flame retardants are a different story. They don’t chemically bond into the polymer the same way. Instead, they sit within the foam matrix and slowly migrate to the surface, where they evaporate into your living space.
Flame Retardants Can Off-Gas for Years
The flame retardant most commonly found in spray foam is TCPP. NIST researchers measured TCPP emissions from both open-cell and closed-cell foam samples and found that open-cell foam released TCPP at a remarkably steady rate. Average concentrations hovered around 300 to 400 micrograms per cubic meter throughout more than 400 hours of testing, with no meaningful decline over time. Even after 18 months of aging, open-cell foam samples continued emitting TCPP at measurable levels when wall temperatures were elevated.
Closed-cell foam behaved differently. TCPP concentrations dropped exponentially during the first 100 hours as the chemical near the surface escaped, and then leveled off at much lower rates. The denser cell structure limits how quickly TCPP from deeper in the foam can reach the surface.
Temperature plays a major role. TCPP emissions are exponentially dependent on temperature, meaning even a modest increase in building heat can cause a relatively large jump in airborne concentrations. In a test home with open-cell foam sealing basement rim joists, raising the basement temperature from about 21.5°C to 28.5°C increased average TCPP levels by 3.4 times. Rooms closest to the foam (basements, attics) tend to have significantly higher concentrations than the rest of the house.
When Improper Curing Creates Real Danger
The more serious toxicity concerns come from foam that never cured properly. If the two chemical components are mixed at the wrong ratio, applied too thickly, sprayed at incorrect temperatures, or applied to damp surfaces, portions of the material can remain reactive indefinitely. In those cases, free isocyanates and other volatile compounds continue releasing into indoor air long after the installation should have been “done.”
A study examining 13 adults from 10 households with improperly installed spray foam documented a consistent pattern of symptoms. All subjects reported a fishy odor along with watery and burning eyes, burning nose, sinus congestion, throat irritation, cough, and chest tightness. Twelve of the 13 also experienced neurological symptoms: headaches, dizziness, forgetfulness, difficulty concentrating, and insomnia. These were not brief reactions. The study described both acute and persistent symptoms tied to the faulty application.
In these homes, residents either were never told to leave during application, were told to return too soon, or the foam itself was simply mixed incorrectly. The problem was not that spray foam is inherently toxic after curing. The problem was that the foam never truly cured.
How to Spot a Bad Installation
Properly cured spray foam should be firm, uniform in color, and odorless within a few days of installation. If something went wrong, the signs are usually noticeable:
- Smell: A lingering chemical or sweet odor that persists beyond the first few days is the most common warning sign.
- Texture: Foam that remains soft, tacky, or crumbly when touched or cut has not fully reacted.
- Appearance: Uneven coloring, glossy patches, or large irregular cell structures suggest off-ratio mixing.
Moisture can also interfere with curing. Foam sprayed onto damp surfaces or exposed to high humidity can trap water, triggering secondary chemical reactions that intensify odors and prolong off-gassing. Heat from roofing materials in attic applications can accelerate the breakdown of improperly cured foam, releasing additional volatile compounds.
Isocyanate Risks After Curing
For most people living with properly cured spray foam, isocyanate exposure is not a practical concern. The chemicals are locked into the polymer. But there are two important exceptions.
First, curing can proceed more slowly than expected. Research has shown that unbound isocyanate species can remain on surfaces for days to weeks after the material appears hardened, particularly in cooler conditions or with thicker applications. The common assumption that isocyanates are gone the moment foam looks solid is not well supported by published data.
Second, heating, cutting, grinding, or sanding cured spray foam can release free isocyanates back into the air. This matters during renovations or any work that disturbs the insulation. Anyone who has developed isocyanate sensitization, which can happen after repeated occupational exposure over months to years, may react to extremely low airborne levels. For sensitized individuals, even trace amounts can trigger asthmatic responses.
Re-Occupancy Times and Air Testing
Most manufacturers recommend vacating the building for at least 24 hours after professional two-component spray foam is applied. Smaller consumer cans (the kind you buy at a hardware store) generally need 8 to 24 hours to cure. These timelines assume correct application under proper conditions, and the EPA notes that they can vary by product and scenario.
The EPA has acknowledged that the potential for off-gassing of volatile chemicals from spray foam is not fully understood, and that standardized methods for assessing indoor air impacts are still needed. If you suspect a problem, whether from a persistent odor, symptoms that started after installation, or visible signs of poor curing, indoor air quality testing through a certified lab can measure specific compounds like TCPP, isocyanates, and other volatile organics in your home. This is especially worth considering if you have open-cell foam in a hot attic or poorly ventilated space, since both foam type and temperature directly affect emission rates.
For homes where the foam was installed correctly and has had adequate time to cure, the primary long-term exposure concern is flame retardants rather than isocyanates. Whether that level of exposure poses health risks over decades of living in the home is still an open question, but good ventilation in spaces adjacent to spray foam, particularly basements and attics, reduces airborne concentrations meaningfully.