Formaldehyde is everywhere. Developing an allergy to it is common for workers in certain industries such as clothing and printing; it's also known to be a carcinogen. If you have particleboard, fire retardant sofas and carpets, read magazines and newspapers, wear clothes with fabric finishes, use toiletries or cosmetics and use standard household cleaning products you will be exposed to formaldehyde.
I'm allergic to it myself, although my allergy is at the milder end. Avoiding it is very difficult.