The following schematic diagram shows the workings of your heater box, where air is drawn in from outside of the vehicle by means of Heater blower motor and is forced into the heater box situated behind your dashboard, the red lines indicate the housing of your heater matrix and air conditioning matrix (Evaporator). The air that is drawn into the car from outside contains atmospheric pollution and pollen. Most of these are trapped through the pollen filter which is situated before or after the blower (depending on the design of the vehicle), but a small amount is passed through the filter which then lodges partly inside the fins of the evaporator and heater matrix and others stick to the plastic interior of the heater box. Through time these pollens along with the pollution find the moist atmosphere of the heater box ripe to grow and create pollen dust that flows through the box into the interior of the car irritating the conditions such as asthma in the occupants of the vehicle. The remaining parts of the pollen and pollution give rise to the growth of the bacteria which in return cause the smell in the car.
There is no remedy to remove this unwanted feature of the system from the car by ratio of 100% , but by taking the following three steps you can ensure that a large percentage is eradicated and therefore, reducing the risk of illness and smell.
1- change the pollen filter regularly and ensure that the filter is fitted correctly to sit tight into its space.
2- wash the interior of the heater box with designated chemicals, this requires skill to access the interior of the heater box to spray the chemicals to the centre of the box, do not waste your time and money buying products that are left inside the car, and claims to do the same work, they are merely, glorified air fresheners.
3- when within a high pollution area, try to use the re circulation facility of your air system as much as possible, this feature comes with practically every car now days, which shuts the air feed to the blower from outside.