大打折扣 (中翻英)?

Chalk dust harms human respiratory tracts, which makes our health greatly worse. Whereas erasers are indispensable for students' life for cleaning up the blackboards after classes, the dust may be everywhere on their clothes during the cleaning.

粉筆灰對人的呼吸道不好，使我們的健康大打折扣.
The dust of chalk is harmful to our respiration system, and of course, produces significantly negative impacts on our health.
板擦對於學生的生活是不可或缺的，在下課後清理黑板時，可能為了清潔黑板讓衣服都是粉塵
It’s students’ duty to erase the blackboard after class, but while doing this, the dust of chalks, formed while cleaning the blackboard, might let clothes stained with chalk dust.

The residues of chalk are a kind of powder which are harmful to our respiratory system and in turn our health will be affected considerably;in addition, it is a usual way in classroom after class dismissed and student will wipe off the drawings on the blackboard so that his face and clothes are covered by lots of white powder. Yip