As part of his “Toxic America” series, CNN’s Dr. Sanjay Gupta has been investigating schools, and his findings are not good. Research suggests that at least one-third of U.S. schools have mold, dust and other indoor air quality issues that can cause asthma in students and staff. It gets worse: According to Healthy Schools Network, as many as 5 million U.S. children may be attending K-12 schools where poor air quality, hazardous chemicals and other unhealthy conditions can make everyone sick. And it isn’t just indoor air quality that’s the problem — the outdoor air at many schools is toxic enough to cause asthma and cancer.