Inhaling thick dust does break your fast, such measures as covering your mouth should be taken when using or cleaning such objects as dust boards. Dust by itself does not break your fast, but rather thick dust, which is found in large concentrations within the air. This is the case as such dust enters your digestive system, similar to medication that that is taken through the anus as it is entering the inside of your body. another example is the use of insertion of a camera through the oesophagus of a person, this would break that persons fast, although it is not food and it is not digested by the body.