The off-gassing issue with MDF (medium density fiberboard) comes from the use of urea-formaldehyde as a binder holding the fibers together. Your symptoms can indeed be caused by formaldehyde, but you should keep in mind that they also may arise from other contaminants or germs.
Formaldehyde off-gassing does appear to have a relationship to indoor conditions, but should actually decrease in colder temperatures. According to the FDA, "Formaldehyde tends to double its level of outgassing for every 10 degree F increase in temperature." Humidity can also have an effect.
Some questions about your MDF:
- Is it exposed, or is it painted, sealed or laminated over?
- How long has it been there? Formaldehyde off-gassing will diminish over time. If it's new, does its introduction coincide with your symptoms?
- Do you know whether your particular MDF contains formaldehyde or not? There are several manufacturers who now produce MDF with "no added formaldehyde."
Ideally, you'd try to determine if the MDF is indeed the cause of the problem. If the wood is relatively new and if it is exposed and unsealed, the US Consumer Product Safety Commission recommends sealing the surfaces: "You may use a vapor barrier such as some paints, varnishes, or a layer of vinyl or polyurethane-like materials. Be sure to seal completely, with a material that does not itself contain formaldehyde. Many paints and coatings will emit other VOCs when curing, so be sure to ventilate the area well during and after treatment."
Another part of the remedy may be "dilution" by increasing the ventilation in the space. That doesn't get rid of the cause of the problem, of course, and also runs the risk of increasing your heating and cooling loads, but may improve your home air quality and alleviate some of your symptoms.