In article 38772668-883d-4bd8-b6e0-fa81a6092d01
@z27g2000prz.googlegroups.com, says...
Or, in the case of many cars, 'cos the driver doesn't understand how to
set the fresh/recirculate lever.

Vauxhall seem to be to be particularly prone to this - the number of
Corsas and Astras you see driving along that look like someone's having
a sauna in 'em.

if with aircon - the driivers should check their aircon plenum
chambers (IME)

Mostly just plain ordinary ventilation - but switched off.

--
Skipweasel - never knowingly understood.