Forced air heating ducts have many grooves, joints and corners that allow dust, pollen, mould, feathers and animal hair to build up. This dirt is the main cause of allergic reactions, due to dust and spores being blown out of the heater into the home. To limit this, a forced air heating system should be fitted with a good HEPA filter. HEPA is short for High Efficiency Particulate Air filter. These kinds of filters were originally developed to prevent radioactive contamination of personnel working on the Manhattan Project in the 1940's. These filters have different efficiency ratings and can almost block all dust and allergens from passing through.
The filter must be fitted properly according to manufacturer's instructions and must be replaced at the recommended intervals
Ducting should be cleaned out at least once a year and definitely before the start of the cold season. Sometimes it is better to have this done professionally.