Spray foam insulation contains chemicals that are genuinely toxic during application and curing, but once fully cured, it is considered relatively inert. The key variable is timing: the hours and days around installation carry real health risks, while the long-term picture depends on whether the foam was mixed and applied correctly.
What Makes Spray Foam Toxic
Spray polyurethane foam is created by mixing two chemical components, often called side A and side B, in roughly equal parts. Side A contains isocyanates, a family of highly reactive chemicals that are powerful irritants to the eyes, skin, and respiratory tract. Side B contains a polyol (which reacts with the isocyanates to form polyurethane) along with catalysts, blowing agents, surfactants, and flame retardants.
Isocyanates are the primary health concern. They can irritate the airways so severely that the irritation progresses to chemical bronchitis with intense constriction of the airways. More importantly, isocyanates can sensitize your immune system. Once sensitized, even tiny concentrations of isocyanates can trigger a severe asthma attack. NIOSH, the federal agency that researches workplace safety, has documented deaths from severe asthma in sensitized individuals, and no safe concentration has been identified for people who have already been sensitized.
Beyond isocyanates, spray foam contains a flame retardant called TCPP (tris(1-chloro-2-propyl) phosphate), which can make up to 12% of the foam’s weight. TCPP increases cell toxicity in lab studies and affects fetal development. It is persistent in the environment and absorbed through the skin. Open-cell spray foam generally contains higher concentrations of TCPP than closed-cell foam.
Risks During and Right After Installation
The most dangerous period is during spraying and in the hours that follow. Professional spray foam is applied at 1,250 to 1,900 psi, producing a cloud of aerosol droplets that carry isocyanates and flame retardants into the air. During application, TCPP air concentrations for sprayers average around 87 micrograms per cubic meter, hundreds of times higher than the levels found in normal indoor air (typically under 0.26 micrograms per cubic meter). Isocyanate vapors, mists, and particulates fill the work area at the same time.
This is why professional installers are required to wear full protective gear: appropriate respirators, chemical-resistant gloves, and chemical-resistant clothing. No one should be in the building during installation without that level of protection. Workers even need respiratory protection when cutting or scraping cured foam, because the dust can contain isocyanates.
After the foam is sprayed, it may look hard and feel dry to the touch within seconds or minutes. That does not mean it is safe. The foam is still actively curing and still releasing unreacted chemicals. Most manufacturers recommend that residents stay out of the building for at least 24 hours after professional two-component foam is applied. For single-component foam sold in small cans, curing takes roughly 8 to 24 hours. The actual time varies with the product formulation, the thickness of the application, temperature, and humidity.
What Happens When Foam Is Applied Incorrectly
A properly mixed, properly applied spray foam job is one thing. A botched job is quite different, and the consequences can be severe enough to force people out of their homes.
A study of residents exposed to improperly applied spray foam found a consistent pattern. In homes where the foam was mixed at the wrong ratio or applied without proper ventilation, all residents reported a fishy odor along with watery, burning eyes, a burning nose, sinus congestion, throat irritation, coughing, and chest tightness. Over 90% developed neurological symptoms: headaches, dizziness, difficulty concentrating, forgetfulness, and insomnia. About a quarter experienced nausea, vomiting, and abdominal cramps. Another quarter developed skin rashes.
These were not short-lived reactions. Symptoms persisted long after the installation was complete. They would subside when residents left their homes and return when they came back. Air testing showed elevated concentrations of volatile organic compounds derived from the spray foam. The symptoms only resolved after the foam was physically removed from the homes. Every resident in the study eventually had to vacate.
Is Cured Spray Foam Safe Long-Term?
The EPA considers fully cured spray foam to be “relatively inert,” which is an important qualifier. When the chemical reaction between the two components goes to completion, the isocyanates are locked into the polyurethane matrix and are no longer releasing vapors at meaningful levels. For most correctly installed spray foam, this means it does not pose ongoing isocyanate exposure risks to people living in the building.
There are caveats. Cured foam can still pose risks if it is cut, sanded, or disturbed in a way that generates dust, because those particles may contain unreacted isocyanates. The flame retardant TCPP is a separate concern: it is semi-volatile and persistent, meaning it can slowly migrate out of the foam over time. Residential air measurements have detected low levels of TCPP in homes and offices, though these concentrations are far below what installers experience during application. Whether those low-level, long-term exposures matter for health is less settled than the acute risks.
Open-Cell vs. Closed-Cell Foam
Both types contain isocyanates and carry the same acute risks during installation. The main difference relevant to toxicity is that open-cell foam generally contains higher concentrations of TCPP flame retardant. Open-cell foam is also softer and more porous, which may allow more off-gassing of residual chemicals over time compared to the denser, more rigid structure of closed-cell foam. Both types require the same protective measures during installation and the same re-entry waiting period.
How to Reduce Your Risk
If you are having spray foam installed in your home, the most important steps are straightforward. Leave the building during installation and do not return for at least 24 hours, following whatever timeline the manufacturer specifies. Make sure children, elderly family members, and pets are out as well. Verify that your installer is experienced and using properly calibrated equipment, since the ratio between the two chemical components must be precise for the foam to cure completely.
If you notice a persistent chemical or fishy odor after returning to your home, take it seriously. That smell suggests the foam did not cure properly and may be releasing unreacted chemicals. Ventilate the space and contact the installer. In documented cases of faulty application, the only effective solution was complete removal of the foam.
For small DIY projects using single-component cans (the kind sold at hardware stores), the same chemical hazards apply on a smaller scale. Work in a well-ventilated area, wear gloves, and avoid breathing the fumes. These cans contain the same isocyanate chemistry, just in smaller quantities.
People with existing asthma or other respiratory conditions face higher risk. Isocyanate sensitization is unpredictable, but once it develops, it is essentially permanent and can be triggered by exposures too low to measure with standard workplace monitoring. Animal studies have also linked one type of isocyanate (TDI) to cancer, meeting OSHA’s criteria for classification as a potential occupational carcinogen, though this is primarily a concern for workers with repeated, long-term exposure rather than homeowners.