Over 80 percent of people living in urban areas monitoring air pollution are living with poor air quality levels that exceed limits set by the World Health Organisation (WHO).
In a news release on Thursday, the WHO added that 98 percent of cities in "low and middle income countries" with over 100,000 people did not meet the WHO's guidelines for air quality. For those living in high income countries, this dropped to 56 percent.
The WHO said that a decline in urban air quality would increase the risk of lung cancer, heart disease, stroke as well as "chronic and acute respiratory diseases" such as asthma.
Air pollution was described as a "major cause of disease and death" by Flavia Bustreo, the WHO's assistant director general for family, women and children's health.
"When dirty air blankets our cities the most vulnerable urban populations—the youngest, oldest and poorest—are the most impacted," Bustreo went on to add.
As well as being potentially deadly, air pollution also has a significant financial cost. A recent report from the Royal College of Paediatrics and Child Health and the Royal College of Physicians said outdoor air pollution caused roughly 40,000 deaths in the U.K. every year and cost the U.K. economy more than £20 billion ($28.9 billion) annually.
The WHO also said that between 2008 and 2013, global "urban air pollution levels" rose by 8 percent, with high income countries experiencing, in general, the lowest levels, and low and middle income countries experiencing the highest.
The WHO went on to say ambient air pollution – which consists of "high concentrations of small and fine particulate matter" – caused over three million early deaths annually.
"It is crucial for city and national governments to make urban air quality a health and development priority," the WHO's Carlos Dora said.
"When air quality improves, health costs from air pollution-related diseases shrink, worker productivity expands and life expectancy grows. Reducing air pollution also brings an added climate bonus, which can become a part of countries' commitments to the climate treaty," Dora added.