Toxins in some construction materials can be quite toxic. One not mentioned yet is vinyl (flooring/ walls). Also, formaldehyde is in some wood products. Glad to hear you are cleaning your ductwork, as all sorts of things can be in there causing the problem...especially after some remodelling. That is where I would start. If things do not improve quickly after the ducts a re cleaned, it's possible some the of the construction materials (or furniture) are offgassing. Here is a list of things you should be suspicious of:
Paints, Human by-products, Cleaning Products, Glues and Sealants, Carpets, Dust Mites, Combustion by-products, Radon, Smoke, Solvents, Moisture, Pollen, Lead, Mold and Mildew, Bacteria and Viruses, Fiberglass, Pesticides, Pets,
Attached Garages (carbon monoxide), Furniture, Printers/ copiers
Top Offenders – and how to avoid them
The Problem:/ The Solution:
--Urea formaldehyde sheet goods/ Use only sheet goods (plywood/ particleboard) that are labeled as No Added Formaldehyde (NAF)
--High VOC carpets and carpet backing/ Use Carpet and Carpet Backing certified by the CRI’s “Green Label” program
--Improper material handling and protection/ Keep building materials dry
--Subfloor Adhesive, Tile and carpeting Adhesives and Mastics/ Use Adhesives and Mastics that have third party certification such as GreenGuard and GreenSeal
--Vinyl wall and floor coverings /Avoid using Vinyl wall and floor coverings
--High VOC Floor Finishes (such as Glitza)/Use Pre-finished products, Oil and Wax Finish, or Water Based Polyurethane
--Regular paints and stains / Specify low or No VOC products