SINGAPORE - Lockdowns and movement restrictions have led to blue skies in some parts of South-east Asia, giving residents a respite from air pollution.
The World Health Organisation says hazardous air in the region causes about 800,000 deaths a year and the brighter skies, however temporary, underscore how dependent the region is on fossil fuels for energy and transport and the health risks.
A study released on Friday (May 8) has revealed how much pollution has dropped around the region in recent weeks - and places where it hasn't.
The analysis by the Centre for Research on Energy and Clean Air (Crea) focused on nitrogen dioxide (NO2) pollution and, where data was available, readings of fine particulate matter of 2.5 microns in size (PM2.5), comparing the impact of lockdown periods this year with the same period last year.
For most locations, the measurement period was between mid to late March and May 5 this year.
NO2 is regarded as a good proxy for local air pollution and can be measured by satellites. It is produced by burning fossil fuels, such as the emissions from cars, trucks, buses and power plants.
The United States Environmental Protection Agency said air with a high concentration of NO2 can irritate airways in the respiratory system, aggravating respiratory diseases, particularly asthma.
NO2 along with other nitrate aerosols also reacts with other chemicals in the air to form very fine particulate matter, such as PM2.5, which gets deep into the lungs and can cause premature death from respiratory and cardiovascular disease.
Crea's analysis shows NO2 levels for Kuala Lumpur, Manila and Bangkok fell because of a sharp drop in transport and manufacturing. For example, from March 18 to May 5, Malaysia recorded the most drastic and sustained changes of the places studied, with Kuala Lumpur experiencing about a 60 per cent reduction of NO2 levels compared with the same period in 2019.
For Singapore, there was little change in PM2.5 during the measurement period of Feb 10 to May 5 because much of the pollution comes from power plants, industrial complexes, residential and other sources in surrounding areas, said the authors, lead analyst Lauri Myllyvirta and analyst Isabella Suarez.
NO2 fell by about 30 per cent, except over western Singapore around Jurong Island, where high levels of NO2 persisted.
Earlier this week, in response to queries from The Straits Times, the National Environment Agency (NEA) said key pollutant levels had been falling even before the circuit breaker measures started. They declined further after the measures kicked in on April 7.
The NEA said the average NO2 level in the two weeks before the circuit breaker period was 17 ug/m3 (micrograms, or one-millionth of a gram, per cubic metre of air) - down from 27 ug/m3 in the same period last year.
"Preliminary analysis shows that in the two weeks since the circuit breaker measures were introduced, there was a further improvement in air quality, with the average level of NO2 decreasing to 13 ug/m3," it added.
The agency said the average levels of PM10 and PM2.5, carbon monoxide and sulphur dioxide had fallen by between 8 and 43 per cent. After the circuit breaker measures kicked in, these pollutant levels fell further, but by "less than 1 per cent".
Crea noted that NO2 emissions also fell in Jakarta and Hanoi but PM2.5 levels have remained high because both cities have large coal-fired power plants on their fringes, meaning a sizeable portion of local air pollution is produced outside. The findings were similar for Ho Chi Minh City, too.
Cambodia and Laos, with looser restrictions, recorded little change in pollution readings, the authors found.
They said the findings underscore the urgent need to strengthen air quality regulations and speed up the shift to greener energy.
"Blue skies above our major cities point to what we can achieve if we invest in clean energy as the crisis abates," said Ms Suarez.
"Through the crisis, we have had a glimpse of what life can be like with more breathable air. But turning this into a daily reality can only be achieved by enforcing air quality standards and rapidly reducing fossil fuels in a sustained and sustainable way."