Hi SleepDeprived,
Most mattresses use a barrier system to pass the fire regulations (either a “sock” that goes around the materials in the mattress or a fire retardant barrier that is part of the mattress cover) and generally don’t use fire retardant chemicals in the foam itself. Fire retardant foams are much more common in the furniture industry than the mattress industry.
I don’t think that cutting foam on an occasional basis would be harmful but if you tend to be sensitive or are concerned then I would suggest a mask such as what brass suggested. If you were cutting foam on a more regular basis or working in a factory with a higher concentration of dust in the air then protective measures would certainly make sense. Just to add one more example, I used to work in a grain elevator many years ago and we always used masks when we were doing certain jobs or when the air was dusty because continuous exposure to dust of any type can certainly cause health issues.
Again … it isn’t something that I would be concerned with unless you were doing it on a regular and ongoing basis.
For reference there is more about mattress surgery in post #2 here and the other posts and information it links to.
There is also more about fire retardants in post #2 here.
While each person will have their own answers to these types of questions about “how safe is safe enough for me?” (just like some people are much more careful than others about their dietary choices) … it’s not something that would concern me if I was occasionally doing mattress surgery or cutting foam.
Phoenix