Most installations of spray foam insulation, when properly installed, act as an air barrierBuilding assembly components that work as a system to restrict air flow through the building envelope. Air barriers may or may not act as a vapor barrier. The air barrier can be on the exterior, the interior of the assembly, or both.. When you use it instead of the fluffy stuff (fiberglass, cellulose, cotton), a house will be more airtight. That's good.
When a house is airtight, the nasties in the indoor air tend to stick around. Volatile organic compounds (VOCsVolatile organic compound. An organic compound that evaporates readily into the atmosphere; as defined by the U.S. Environmental Protection Agency, VOCs are organic compounds that volatize and then become involved in photochemical smog production.), water vapor, odors, radonColorless, odorless, short-lived radioactive gas that can seep into homes and result in lung cancer risk. Radon and its decay products emit cancer-causing alpha, beta, and gamma particles., and other stuff you don't want to immerse yourself in make the home's indoor air quality worse.