Sick building syndrome is not just something you see in commercial or industrial buildings. Residential buildings can have worse indoor air quality than you see outdoors, even in city environments infamous for their poor outdoor air quality.
Small children, homemakers and elderly or housebound individuals typically spend a great deal more time literally at home than most people do. Children and the elderly are especially vulnerable to the effects of pollution.
Unfortunately, there is no universal answer as to what you can assume to be the single biggest pollutant in your home. This will vary from household to household and depend on a variety of factors. You will need to make an effort to determine which household pollutant is the single biggest one in your house. But you can look to the following categories for likely suspects.
New carpeting can emit a variety of volatile organic compounds (VOCs). This is often called off gassing. The exact chemicals involved will depend upon the material in question. How bad it is will depend in part on how much new carpet has been installed.
Broken compact fluorescent bulbs
These bulbs contain mercury, a known neurotoxin. It is classified as a hazardous material and should be handled accordingly. Do not try to vacuum it up. If it gets into carpeting, you can expect to need to throw it out. It probably cannot be ever fully removed.
New plastic products
The softer the plastic, the more likely it is to off gas. This gets worse if they get warm for some reason, as can happen when you run certain electronics with a plastic casing.
Household cleaners and chemicals
Glues, adhesives, paint, paint stripper, pesticides and cleaning products can all introduce problematic fumes into the air. All should properly stored and properly used with good ventilation, among other things.
Mattresses, upholstered furniture and many pressed wood items can all be sources of air pollutants. New furniture can give off formaldehyde. Old furniture may be a source of mold, dust, mites and other problems.
Cooking and heating
Fireplaces and wood burning stoves tend to be harder on air quality than some other heat sources, but most means to cook or heat have some fallout in terms of air quality. Passive solar design might be the only means to keep the place warm without somehow harming local air quality.
Although there are many known culprits that can harm indoor air quality, there is no universal answer to this issue. An audit of your home is the best way to determine what is the biggest source of indoor air pollution in your own house.