There were actually two reasons why Kazuichi went out to get some fresh air... no, make that three.
First and foremost, he wanted to get some fresh air since the smell of Hybrids underground was getting on his nerves. Quite frankly, he felt like the smell was choking the life out of him, and it wouldn't take long before he went feral due to how intense the scent was.
Secondly, he wanted to eat something. He knew that there might be food being served underground, but Kazuichi wouldn't dare risk his sanity over a bowl of food. Besides, it wouldn't do him good if he were to eat with other Hybrids. It would just trigger his instincts and he might attack someone or slice someone's head off before he realized what he was doing.