Many of the contaminating substances give no warning and may produce symptoms years later. So it's best to control or eliminate exposure now before they cause any harm.
Asbestos was common in roofing, flooring, well and pipe insulation, speckling, cement, coating, heating equipment and acoustics. Generally if it is sealed or undisturbed, it does not pose a threat. But disintegration and other disturbances can cause small fibers to get into your lungs.
Keep humidity low and ventilation high to thwart the dangers of viruses, bacteria, molds and fungi. Wash bedding materials in hot water and keep a clean house.
Monitor all fuel-burning appliances so that carbon monoxide doesn't build up. Install a detector with an audible alarm.
Reduce formaldehyde-producing sources that can be found in some air fresheners, particleboard, fiberboard, plywood paneling, carpet backing, upholstery and drapery fabrics.
Check for nitrogen-dioxide (maintain vents), radon (in the soil surrounding the house), second-hand smoke and toxic chemicals in household products. Pay attention to the warnings. Consider installing a high-efficient air cleaner with either larger filters or an electronic air cleaner. Ventilate your home-open windows when the weather is nice.