On 23:30 16 Sep 2019, alan_m wrote:

Having seen 3 examples in the last couple of days - why in films when
people are crawling around ventilation ducts they are always spotlessly
clean and when emerging those wearing black clothing show no signs of
any dust?

Similarly ...

Car windscreens in films are often spotless, even after miles of driving.

Wooden floors are beautifully dust free, even though no one is ever seen
cleaning them.