In episode three when Grievous is in space he broke out of the invisible hand by jumping into space. Him being half droid I didn't question it, but when I thought about it I realized. Grievous is coughing. You can't cough if you have no lungs. So I looked in the databank and it said Grievous does have lungs. So Grievous couldn't breath in space. It was one of the few things he didn't have replaced with geonosian armour.