The first and most important step in making any home a healthier place to live, whether it's a passive solar design or a traditional house, is eliminating sources of air pollution. The problem with eliminating sources is that many of them are built into the fabric of the building. It's a lot easier to remove those pollutants if you're working on a new build than a retrofit.
Appliances that burn fuel
Every appliance that uses combustion produces air pollutants that can end up inside your home. Furnaces, water heaters, wood stoves, gas ranges, and fireplaces all do it. The worst dangers come from carbon monoxide and nitrogen dioxide. These poisonous gases cause sickness and can potentially kill. The "greener" appliances, such as wood stoves and fireplaces, also produce particulate matter (soot and so on).
The only way to remove the pollutants produced by combustion appliances is to use electrical equivalents. However, this makes them less efficient, less friendly to the environment, and a lot more expensive in the long run.
The best option, therefore, is to choose appliances which are sealed – water heaters, boilers, and furnaces with sealed combustion chambers and which take air from outside the house. If you're retrofitting without replacing existing appliances, adjust your usage for a cleaner burn.
Carpeting is a horrific pollution-trap. It holds dust, dust mites, other allergy-provoking particles, mold, mildew, and dander. And that's without counting air pollution from combustion appliances, chemicals from detergents, and whatever else drifts around your living spaces. All of those substances get absorbed and outgassed over long periods. On top of that, most new carpets contain formaldehyde, which is released into the home for weeks or months.
Eliminate or reduce carpet wherever you can. Ceramic tiles, plaster, hardwood, and wool are all better options for finishing walls and floors; natural linoleum, bamboo, or cork are even better for floors.
Engineered wood products contain chemicals that will be outgassed into the home long after construction is complete. This is a harder cutback because engineered wood products, such as plywood, particle board, and oriented strand board, are all made with leftover bits of wood (sawdust, chips, thin strips, etc.) and thus reduce environmental impact. However, all those bits are stuck together with resin that contains formaldehyde.
If you can't get non-engineered alternatives, target products that have reduced formaldehyde content, or even natural products such as particle board made from straw.
Conventional, traditional paints contain a frightening number of poisons. Most basic commercial products still contain formaldehyde, pesticides, anti-mildew chemicals, solvents, and other volatile organic compounds (VOCs) that will be released into your home for months after work is complete.
Once again, this is a tougher choice. Firstly, because some of the chemicals in paint have a purpose – notably the anti-mildew and fungicide products; secondly, because some of the low-VOC alternatives contain traces of mercury and lead.
Choose paints which are as "clean" as possible: the Safecoat and BioShield ranges are both good. The former costs more but is free from formaldehyde, fungicides and mildewcide, as well