Are crowded subways a health risk?

Unpredictable. From a practical standpoint, sure if there's a person with a cold standing in a crowded subway, there is an increased risk that person's sneeze and germs will find its way to another person. In terms of personal health, can't really say there's any added risk of riding the subway. Public transport is good for the environment. You could argue, driving a car is more dangerous (ie accidents).