HOUSTON—Bryan Parrasstood in the shadows cast by glaring floodlights ringing the massive white, cylindrical tanks of the Valero oil refinery. He, like many other poor Mexican-Americans who grew up in this part of Houston, struggles with asthma, sore throats, headaches, rashes, nosebleeds and a host of other illnesses and symptoms. The air was heavy with the smell of sulfur and …