Something wrong with the Air

[Jerry Limber]

All,

I am in the middle of rehabbing my first investment property and think my house is experiencing some kind of air quality problem and I'm not sure how to proceed.  My partner stayed in the house for a few days and as a result developed a pretty bad cough which seemed to go away and feel better as soon as he left.  We had a couple of friends stay in the house for a couple nights (different rooms) and they also began coughing.  Last night I spend about 30 minutes at the property and by the end of it felt a slight scratchiness in my throat and coughed a bit for the rest of the evening.  

We had someone come and do a mold inspection that included an air quality assessment but spore counts were at normal levels throughout the house with the exception of the one room that is still under construction that no one is staying in.  

The AC and ductwork are both brand new, but the house was originally built in the 50's.  Our contractor suggested that lead in the plumbing or paint might be a problem?  Since it isn't mold I'm honestly not sure what to check for next?

Thanks in advance for any help you can provide.