An average human can sustain without food for more than 20 days, without water for 3-4 days and without air for 3 minutes. 

So lack of air can kill us faster.

The fuels which leave a lesser amount of toxic substances and undesirable harmful exhausts after combustion are cleaner fuels.
Natural gas is lighter than air and disperses quickly when released. Therefore CNG is a cleaner vehicle fuel than kerosene, petrol and diesel and it is also safer than other fuels. 

Pure air is composed of 78% nitrogen gas, 21% oxygen gas, and 0.03% carbon dioxide. Some other gases like argon, ozone, and water vapours are also present in small quantities. When this composition of air is disturbed by the addition of harmful substances or toxic gases such as nitrogen dioxide, sulphur dioxide, carbon monoxide, and particulate matter, then the air is said to be polluted.

Reverse osmosis often referred to as RO, is an advanced water purification method that was initially developed to produce drinking water from seawater for submarine crews. Modern reverse osmosis units effectively reduce the “dissolved solids” content of water. It is a membrane filtration technology that works by pressing water through the very tiny pores of a semi-permeable membrane to produce a highly purified and great tasting water.

The main reason for water contamination are the wastes from industries and the sewage. Industrial waste includes any material that is rendered useless during a manufacturing process such as that of factories, industries, mills, and mining operations.Sewage waste includes household waste and waste from local businesses.

Air pollution in New Delhi is mostly comprised of pollution from human activities.
Delhi has more than 7.4 million vehicles on its roads, with an additional 1,200 added each day leading to the increase in emission of pollutants. 
Stubble burning by farmers in neighbouring states has also been a contributor to the city's misery.

Hydrogen sulfide is a chemical compound with the formula $H_{2}S$. It is a colorless gas with the characteristic foul odour of rotten eggs. It is heavier than air, very poisonous, corrosive, flammable, and explosive.

Greenhouse gases are the gases in the atmosphere that are capable of absorbing the infrared radiations from the earth, thereby trapping the heat in the atmosphere. This trapped heat is increasing the global temperature leading to global warming and this phenomenon is known as the greenhouse effect. Examples of greenhouse gases are methane, water vapour, carbon dioxide.