Air pollution is at high levels in many parts of Asia. UNICEF estimates more than 100 children under the age of 5 in East Asia and the Pacific die every day from causes linked to toxic air.

The UN agency's analysis says pollution is related to nearly one in four deaths of children under 5 in the region, making it the second-biggest fatality risk after malnutrition. It says 500 million children are living in places where the air is "unhealthy," a situation it dubbed "the silent killer."

The region's air quality typically plummets during the dry season. Vehicle exhaust and industrial emissions are believed to be among major contributors. Children are also exposed to household pollutants released during cooking and heating.

Nicholas Rees, UNICEF Regional Climate Manager for East Asia and Pacific, warns that the impact of pollution exposure can linger into adulthood.

He said: "Air pollution has a huge impact on children's life and their well-being, not just their health, although that is a critical and that's one of the main impacts, but also their learning and hence their prospects. Their prospects for their future."

Pollution levels have been spiking in Vietnam's capital Hanoi. A Swiss-based air quality watchdog says the city ranked among the worst in the world this week.

In Thailand, air pollution forced hundreds of schools to close in the capital last month, and dense smog continues to blanket parts of the country.