Whether or not I'd worry about off-gassing would depend on how old they are. I'd find out more specifically what they are made from. There is a big difference between things like advantec, OSB(chipboard),and Particle board which is what I usually think of when someone says press board(either that or MDF). Neither MDF or Particle board can take any amount of moisture and even OSB breaks down pretty quickly. Advantec or something equivalent would be the only one I would think would last and that stuff ain't cheap. It's also VERY heavy. So I guess it comes down to your situation. Are they all you can afford, if so maybe they'll be ok long enough for you to build or pick up some better used equipment. Otherwise I wouldn't bother with them.