Early voting for District 1’s runoff election is now underway.The race will decide who fills the county supervisor seat that represents much of San Diego’s South Bay. The candidates in this summer’s run-off are two South Bay mayors: Imperial Beach Mayor Paloma Aguirre and Chula Vista Mayor John McCann.On Midday Edition Tuesday, we do a rundown of the race and where the candidates stand on issues like housing, immigration and the Tijuana River sewage crisis.Plus, a recent study from UC San Diego found that pollution from the Tijuana River is also making its way into the air. We hear about the most recent findings and what it could mean for the region.Guests:Kori Suzuki, South Bay and Imperial Valley reporter, KPBSJonathan Slade, assistant professor of chemistry, UC San DiegoAdam Cooper, science and technology fellow, California Council on Science and Technology