Over 99 per cent of India's population is breathing air that exceeds the World Health Organisation's health-based guidelines with respect to PM2.5, a report by Greenpeace India said.

According to the key findings in the report titled "Different Air Under One Sky", the greatest proportion of people living in India are exposed to PM2.5 concentrations more than five times the WHO annual average guideline.

It further said 62 per cent of pregnant women in the country live in the most polluted areas, compared to 56 per cent people in the whole population.

As per the report's annual average PM2.5 exposure analysis, the region with the highest exposure to pollution in the country is Delhi-NCR.

It listed older adults, infants and pregnant women as the most vulnerable groups who are "exposed to worse air".

PM2.5 refers to fine particles which penetrate deep into the body and fuel inflammation in the lungs and respiratory tract, leading to the risk of cardiovascular and respiratory problems, including a weak immune system.

The report said the government "must introduce a robust air quality monitoring system" across the country and "make the data publicly available in real time".

"A health advisory and 'red alerts' for bad-air days should also be issued so that the public are able to take necessary steps to protect their health, and polluters would be required to reduce emissions to protect the environment," the report issued on Friday said.

It said the current National Ambient Air Quality Standards (NAAQS) is "insufficient" and "needs immediate revision".

"The Central Pollution Control Board must set a process of revision of NAAQS based on scientific evidence. The government must ensure the implementation of all the planned activities under the National Clean Air Programme (NCAP)," the report said.

It further stressed that there is an urgent need to make NCAP more transparent, comprehensive and stronger".

"The people are already paying a huge price for the air pollution crisis and it is taking a massive toll on healthcare systems. People are being forced to breathe in polluted air and are faced with a terrible health crisis. They can't afford any delays to act on this crisis," the report said.