Generally avoid them. They can make you sound like you’re shouting! Or you’re angry!!!
If you absolutely do need to use one, make sure it’s only one, not two or more in a row.
And don’t use more than two or three for every 100,000 words of prose. Less is more.
You’re not alone if these trip you up.
We're not a fussy bunch, but just so you know...
No space between the number and the measure such as 10kg, 800g, 9lbs, 7oz.