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. 
Lead in your drinking water. 
Non stick teflon when used on high heat has know to kill indoor pet birds. 
"Common indoor plants may provide a valuable weapon in the fight against rising levels of indoor air pollution. Those plants in your office or home are not only decorative, but NASA scientists are finding them to be surprisingly useful in absorbing potentially harmful gases and cleaning the air inside modern buildings." 
"NASA research has consistently shown that living, green and flowering plants can remove several toxic chemicals from the air in building interiors. You can use plants in your home or office to improve the quality of the air to make it a more pleasant place to live and work – where people feel better, perform better, any enjoy life more." 

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