Have you ever had to film where it is cold. But really cold, I mean...
And that is, below -15/-20 °C.
If it should happen, do not forget to put among each piece of your equipment also a small brush with soft bristles, to always keep at your fingertips.
In fact, in case something should fall on the glasses or on the metal parts of your photo/videocamera (such as snowflakes, gravel, or simply a lint), the biggest mistake you could make would be to try to remove it by blowing on.
The water vapor in your breath, reaching the chilly surfaces, would freeze instantly, with all the negative consequences you can imagine.
With the brush, instead, problem solved!