Why this city is hard on interiors
Three conditions coincide in Kolkata for most of the year, and together they are close to ideal for microbial growth.
Humidity. Relative humidity sits high for months, and peaks through the monsoon. Anything damp inside a car stays damp.
Heat. A closed car in Kolkata sun reaches temperatures well above ambient. Warm and damp is the combination that matters.
A sealed, dark cabin. Cars spend most of their lives shut. The air-conditioning evaporator, behind the dashboard, is permanently dark and regularly wet with condensation.
An air freshener does not address any of those three conditions. It addresses your perception of the result, for about a day.
Where the smell actually comes from
The AC evaporator
This is the most common source and the least accessible. The evaporator condenses moisture out of cabin air; in humid conditions it is wet every time the AC runs. Left wet in the dark, it grows a biofilm, and every subsequent use blows the smell through the vents.
The habit that prevents it costs nothing: two or three minutes before you park, switch the AC off but leave the blower running on fresh air. The airflow dries the evaporator.
Carpet underlay and mats
Water gets in on shoes, umbrellas and through open windows during a shower. The carpet surface dries; the underlay beneath it does not, for days. Rubber mats trap water against the carpet underneath them, which is worse than no mats at all if they are never lifted.
Spills and food
Organic matter in warm, humid conditions behaves exactly as you would expect. A missed spill under a seat is a reliable source of a smell nobody can locate.
The boot
The most neglected space in any car. Wet sports kit, groceries, a damp spare wheel well after monsoon driving — and the boot is connected to the cabin air space.