Ambient (outdoor) air pollution in both cities and rural areas was estimated to cause 4.2 million premature deaths worldwide per year in 2016; this mortality is due to exposure to small particulate matter of 2.5 microns or less in diameter (PM_2.5), which cause cardiovascular and respiratory disease, and cancers.

91% of the world’s population live in places where air quality exceeds WHO guideline limits.

What makes Air outside so polluted:

• Particulate matter - Particulate matter, or particle pollution, refers to a mix of very tiny solid and liquid particles that are in the air we breathe. It is caused by traffic, forest fires, wood smoke.
• Ozone- It is usually called "smog," .Ozone is harmful to breathe, it can cause chest pain, shortness of breath, coughing and throat irritation.
• Nitrogen oxide - Nitrogen dioxide is a gas that comes from burning fuels.
• Sulphur di oxide - Sulfur dioxide is produced by the burning of sulfur-containing fuels such as coal, oil, diesel.
• Carbon monoxide - Carbon monoxide is another gas that forms from burning gasoline, wood, propane, charcoal, or other fuel.
• Toxic air pollutants - Nearly 200 other pollutants emitted into the air can cause diseases, including lung cancer, or cause other harmful effects.

Health effects of outdoor air pollution:

• Cardiovascular disease: It triggers arrhythmias, cardiac failure, and stroke in the short and long term.
• Respiratory disease: It exacerbates asthma and chronic obstructive pulmonary disease.
• Reproductive: Studies have shown that outdoor air pollution causes Increased incidence of low birth weight, preterm birth.
• Mental health: Researchers at the University of California claimed to have found a connection between exposure to air pollution and Alzheimer's disease. Researchers say that short-term exposure isn't safe either , after they found evidence of a connection between cognitive impairment and exposure to pollution through forest fires, charcoal grills, gridlocked traffic and other temporary sources.