Air conditioning removes moisture from cabin air. In warm weather, condensation forms around the evaporator and normally drains beneath the vehicle. Clear water below the engine bay after using the system is therefore often harmless; oily, coloured or persistent fluid should be checked by a workshop.

Some moisture remains around the evaporator housing. Together with dust and organic particles drawn into the ventilation system, this can create conditions in which microbial growth develops.

A damp or musty smell when the fan starts is a common warning sign. It may come from the evaporator, a saturated cabin filter, blocked drainage or moisture elsewhere in the vehicle, so the source should be diagnosed rather than assumed.

People with allergies or respiratory sensitivity may find contaminated cabin air uncomfortable. Symptoms while driving should be discussed with a medical professional, while the vehicle itself should be inspected by a qualified workshop or air-conditioning specialist.

Effective service can include replacing the cabin filter, checking the condensate drain, cleaning accessible components and applying an approved disinfecting treatment to the relevant part of the system. A general aerosol released into the cabin may not address the actual source.

Any disinfectant must be compatible with the HVAC system and used according to its current safety instructions. The vehicle should remain unoccupied during treatment and be ventilated fully afterwards.

There is no universal two- or three-year interval for every car. Follow the vehicle manufacturer's maintenance schedule and arrange an inspection when odour, reduced airflow or persistent moisture appears.