Because to cause physical damage to physical beings, you need to interact with them physically in an energetic fashion.

Even primitive people, with the right knowledge, can construct a test for the presence of a hazardous waste source (i.e. literal canaries in coal mines).

There are a number of methods available to 18th century technology which would be able to infer the existence of an energetic hazard even without fully understanding what it is, provided they were somewhat aware of the underlying principles.