Office workers ill, what could the cause be?

you had the building checked, and that is good... but you don't give enough information, is it a new building, an older building, has something in the environment changed? New carpeting, paint, new flooring? There are numerous chemicals that can cause your issues, not to mention the air filtration throughout the building, did they check the filtration systems for molds and mildews? With more than half your workforce out, you obviously need to find out the problem.