Some wear masks on the trail, others don't. Who is right?

I was walking my dog on the LoBo Trail through Niwot, wearing a homemade blue-cotton mask, when a group of three runners came toward me. None wore masks. One coughed about 25 feet in front of me. I hoped my mask gave me some protection, but I wasn't sure. I heard that wearing a mask was mostly to keep my germs out of someone else's lungs.

The cough set me off. I felt like I was put in danger by so...