Common land, queues at the bridge and the air in your car
Mitcham Common covers around 460 acres between Mitcham, Beddington and Croydon, and the greens at Three Kings Piece and the Cricket Green add more. In late spring and summer, grass pollen from all that open space drifts over the roads around it. If you drive the A236 across the common, or live near the windmill or Three Kings Pond, your car's ventilation is taking in pollen whenever the fan is on fresh air.
Traffic brings the other half of the problem. London Road through the town centre is bus-heavy and stop-start, and the Carshalton Road bridge by Mitcham Junction station is a pinch point, with queues building on both sides. Sitting in those queues, the car draws in exhaust from the vehicles in front. A standard filter catches particles; an activated carbon filter also absorbs some of the smell and gases, which helps on the daily commute. It's worth using recirculate in the worst of the queue too.
Many Mitcham cars are parked on terraced streets overnight and collect leaves and grit around the scuttle. When we change the filter, we check the old one for signs of damp, which usually means a blocked drain, and for mould, which means the smell may be coming from further in. We clean out the housing, fit the new filter so it seals and test the fan on every speed before handing back.
The drive couldn't be much simpler. From Beddington Corner we're a couple of minutes down London Road; from the town centre, take Cranmer Road to Carshalton Road and head south over the bridge, around 8 minutes in all. You'll get a written quote first and a 3-month workmanship warranty. It's a quick job, so it's easy to fit in around work or add to a service, and we're open until 8pm, Monday to Saturday.

