ducts, mould and the smell you should not ignore
That musty, gym-bag smell when the AC first kicks on has a specific cause: biofilm. Fine dust that gets past the filter lands on a coil and drain pan that stay wet for six months of the year, and the mixture grows mould and bacteria. The smell is the mildest symptom — black dust streaks around supply vents and indoor allergy flare-ups point the same way, and in a country where people live sealed indoors from May to October, indoor air quality is not a luxury concern.
Duct cleaning and coil cleaning are different jobs; know which you are buying. A chemical coil wash treats the wet heart of the unit — evaporator, blower wheel, drain pan — and fixes most smells; it earns its keep annually. Duct cleaning treats the tunnels: proper firms use rotating brushes or air whips to agitate dust and a negative-pressure vacuum to extract it, section by section. A man with a household vacuum and a torch is redistributing your ducts' contents, not cleaning them. Honest frequency is boring: every two to three years, sooner after renovation (gypsum dust is ductwork's worst enemy) or when moving into a flat with unknown history. Anyone insisting every home needs it every six months is selling frequency, not hygiene.