Fact 1: Amold spore has a negative impact on the respiratory system.

Fact 2: Mold spores, for example, are microscopic.

Given the two facts above, answer the question "What can cause respiratory issues?" with the following options: 
- smoking tobacco
 - a pathogens
 - Microscopic organisms
 - Normal humidity
 - some viruses
 - Clean air
 - smoking pipes
 - Mild weather


 

   Microscopic organisms      