Seems to be caused by the period before the catalytic converter gets up to normal exhaust temperature.

“Vehicles fitted with catalytic converters emit most of their total pollution during the first five minutes of engine operation; for example, before the catalytic converter has warmed up sufficiently to be fully effective.”

https://en.m.wikipedia.org/wiki/Catalytic_converter

Sure. But otoh the car is idling for tens of minutes maybe and possibly no single part of the engine and exhaust system is gaining the needed temperatures to operate cleanly. This means more emissions, more wear and more stressed out neighbours. I'm sure about the last one at least.

There is probably a sweet spot. Certainly, you want to avoid loading a cold engine too much. But don't idle for qn hour either.