There's one thing old houses do a lot better than new ones: They breathe.
You may not think, on a frigid winter evening when cold air is sweeping across your feet, that's such a nifty attribute. But in some ways, it is.
Until the energy crisis of the 1970s forced people to think about conservation, most houses were pretty drafty. To fight the chill, people simply turned up the heat. But when the prices got less tolerable than the cold, builders and homeowners began to tighten up. And that's when the quality of indoor air became an issue.
Houses -- all houses -- collect pollutants. They come from outside sources, from building materials, from dust particles, and from the activities of people and pets. All can cause health hazards, though some are hazardous only to those who are sensitive.
In drafty houses, air inside got changed regularly, and the level of pollutants was generally low. In airtight houses, natural air exchanges have been cut, and the level of pollutants generally has increased.
The sources fall into two categories. There are the Big Three -- radon, lead and asbestos -- which can affect everyone, and all the rest, which affect people in varying degrees.
If you've got one of the Big Three, chances are you've already taken steps to eliminate the hazard. If you haven't, you may need to seek expert help. In some areas abatement actions (especially involving lead or asbestos) are mandated by law.
The other hazards are harder to detect (though there are testing kits for some of them) and the effects may not be so clear. But you can rest assured that some or all of the following are present in your house:
*Chemical compounds: Ketones, aldehydes, formaldehyde and others vaporize from household materials such as adhesives in plywood or particle board (including cabinets that aren't solid wood), furniture, upholstery, carpets, insulation and recently dry-cleaned clothing.
*Pesticides: Chemicals are introduced by use inside or by inadequate storage.
*Volatile organic compounds: Chemicals in paints, hobby materials, waxes and glues, shoe polish, disinfectants, aerosol sprays, solvents and cleaning materials get into the air by use indoors or by inadequate storage.
*Moisture: Although it can come from heating and cooling devices or simple leaks, most moisture comes from humans and animals breathing, sweating, cooking, bathing, cleaning. While not a pollutant itself, moisture can increase outgassing of other pollutants, and encourage growth of bacteria and viruses plus molds, mildew and fungus.
*Combustion contaminants: Furnaces, wood stoves, fireplaces, stoves, clothes dryers can introduce carbon monoxide, nitrogen dioxide, nitric oxide, sulfur dioxide and formaldehyde into the house.
*Smoking: Tobacco smoking can introduce carbon monoxide, nicotine and aldehydes into inside air, as well as particulates such as benzo-(a)-pyrene (a carcinogen). While smokers are exposed to much higher levels, all building occupants share contaminated air.
*Particulates, organic and inorganic: The two biggest culprits are house dust and tobacco-smoke byproducts, but others include pollen and spores, insect debris and pet dandruff.
All this doesn't mean modern house dwellers or rehabbers should move into a tent. But it does mean paying attention to ventilation and eliminating pollution sources.
Next: Improving indoor air quality.
Mr. Johnson is construction manager for Neighborhood Housing Services of Baltimore. Ms. Menzie is Home Editor of The Sun.