FRESNO, California—Inside her single-story home in the dry and dusty Central Valley, Dalia Mondragon scarcely sleeps. Several times a night, she tiptoes into her children’s rooms to make sure their chests are peacefully rising and falling. “I feel like any time they could stop breathing,” she said. Mondragon and all four of her children have asthma—a disease that has sent them to the hospital more times than she can count.