Is there any reason to fear personal exposure to rain with a pH of 3.1?
According to a United States Environmental Protection Agency report on the effects of acid rain on human health:

Walking in acid rain, or even swimming in a lake affected by acid rain, is no more dangerous to humans than walking in normal rain or swimming in non-acidic lakes. However, when the pollutants that cause acid rain $\ce{—SO2}$ and $\ce{NOx}$, as well assulfate and nitrate particles— are in the air, they can be harmful to humans.
$\ce{SO2}$ and $\ce{NOx}$ react in the atmosphere to form fine sulfate and nitrate particles that people can inhale into their lungs.  Many scientific studies have shown a relationship between these particles and effects on heart and lung function, such as asthma and bronchitis.