Especially in higher end custom keyboards, it's common to see foam being used to dampen sounds and vibrations, which can make for a more pleasant sound profile and typing experience.
Foam can be made from a variety of materials, among which is EVA foam. EVA is a material that has various properties which make it suitable for use in keyboards, but it has a caveat: for a while after manufacturing, the material goes through a process called "off-gassing".
What is off-gassing? Off-gassing is the airborne release of chemicals. If you smell a product, it is off-gassing. Various products go through this process, including paints, plastics but also fruits and vegetables.
What are the dangers of off-gassing? Prolonged exposure to certain chemicals can pose danger to your health, and may have serious effects. Not all chemicals pose a risk, and many are harmless.
What about EVA foam? EVA foam off-gasses a chemical called formamide. This chemical, when off-gassed in too large quantities, can pose serious health risks. It can also cause discoloration in certain plastics, the prime example of which is the material of which some of Gazzew's white-colored switches are made.
Can I still use EVA foam? Yes. The European Union posed an emission limit of 20 μg/m3 of formamide for foam and toys containing more than 200 mg/kg of formamide (Directive 2015/2115). If the foam complies with this regulation, it is deemed safe for use. Do mind that discoloration of plastics can still occur even if the foam off-gassing is within the limits set by the directive.
Discoloration of switches usually does not impact functionality or the operable life of the switch.
About formamide. Formamide is a chemical and can be used during the production of plastics. It can be used as a softener and a solvent. When manufacturing foam, it can be blown through the plastic solution to form bubbles within the foam. Remaining formamide then slowly leaves the foam over time, which is part of the off-gassing process. It is known to be carcogenic in certain concentrations , but should not pose particular harm in most keyboard foams due to the relatively small volume of foam. Formamide is molecularly similar to formaldehyde, which is known to be able to yellow plastics and other porous materials.
You may choose to check with your vendor whether the supplied EVA foam conforms to the directive. Please be aware that it may cause discoloration, both when used within the keyboard, as well as when stored nearby switches. Any foam sold through splitkb.com during and after January 2022 will always conform with the directive.