To make a clean and green city, you need the right kind of plants in your indoor and outdoor environments. 

Without plants there are several ways that your environment can make you sick. 
Radon - A gas that comes from wood during changes in temperature and humidity. 
Stray electrical MAGNETIC fields from electrical wires. Use a compass and walk around your home or office. If there is a strong magnetic difference from MAGNETIC north, then that might be the cause. 
Formaldehyde gas from plastics or glue from your carpet or flooring. 
India's research on the most healthiest building is worth looking into.