I worked on PU foam R&D for 8 years so I know a bit about it. Isocyanate is highly reactive and is all consumed in the manufacturing process. Such foams are widely used in cot mattresses and institutional bedding. The combination of mattress cover and condition is highly significent in safety terms. I wouldn't buy a secondhand one in any case. Modern formulations are safe although there have been questions related to cot death and I would not advise using a secondhand one for babies, especially very young ones. There was also govt. advice to lie babies on their backs. Again, I wouldn't buy a secondhand one.
There were also concerns about the extreme flammability of early formulations and much development focussed on dealing with this problem and you would be advised to check flammability ratings on any products you have. Latex foams do not suffer from the same potential disadvantages but again should be checked via manufacturers advice.