Hong Kong(CNN) Sometimes to win the war against pollution, you have to beat a tactical retreat.
That was the message from one of China's top officials Thursday, who warned against taking overzealous measures against coal burning after a clampdown last winter left millions of households across Beijing and northern China without heat during the coldest parts of the year.
Temperatures in the capital can plummet to lows of -15 degrees Celsius (5 degrees Fahrenheit) during the winter months.
Speaking at a symposium in northern Hebei province, which borders the Chinese capital, Vice Premier Han Zheng said it was a government priority to transition to clean energy -- such as natural gas or renewables -- for heating.
However, he said officials must ensure natural gas supplies are sufficient, adding it was "absolutely forbidden" to dismantle existing heating facilities and leave households without a source of warmth.
The announcement comes amid the release of new research suggesting that cutting coal emissions is not necessarily the only way of reducing smog.
Scientists from Harvard and two Chinese universities found that emissions of formaldehyde, rather than sulfur dioxide, played an outsized role in contributing to China's pollution problem, opening up new ways to reduce overall emissions.
In late 2017, China announced a major curtailing of small-scale coal burning -- one of the biggest sources of pollution in winter -- by shifting around 3 million households in northern China to natural gas.
"But over ambitious implementation by local governments took this figure to 5.5 million in reality," according to a Greenpeace report, causing widespread gas shortages and leaving millions without fuel for heating and cooking.
"The large scale push to eliminate small-scale coal burning in provinces surrounding Beijing failed to install homes with gas heaters or pipes in time, leaving them without heating in sub-zero temperatures."
Winter in northern China is bitterly cold, with temperatures falling well below 0 degrees Celsius between November and March, hitting around minus 10 degrees Celsius (14ºF) in some areas.
Coal is by far the main source of heating in the country, providing 83% of heat in 2016 according to Greenpeace, so by cutting this off, many households were without alternative sources of fuel.
The need to cut indoor coal emissions is extremely important, with the World Health Organization estimating that millions of premature deaths each year are caused by indoor air pollution created by the burning of solid fuels such as coal.
Under China's current anti-smog plan, over 3.6 million households in Beijing, Tianjin and 26 other cities in northern China will have had their coal-fired heating systems converted to healthier alternatives by the end of this month.
But the desire to avoid a repeat of last winter's crisis will see a more gradual shift away from coal for many other areas.
Nor is heating the only thing driving a reversal in zealous anti-smog policies. According to the South China Morning Post, targets for emissions reductions across northern China have been revised down from around 15% last year to between 3 and 5% this winter.
While an official explanation has not been given, experts speculated that economic slowdown -- exacerbated by an ongoing trade war with the US -- could have left officials less willing to take drastic measures such as shutting down construction sites or factories.
"Last year's industrial production curbs were fairly successful in reducing air pollution without damaging the economy, (but) there was intense public criticism when other policies overreached, said Rosealea Yao, an analyst with Gavekal Dragonomics in Beijing.
She noted that officials will not want to suffer the "embarrassment" of fuel shortages again, "they are also likely to be more concerned about the potential impact to growth than they were last year, with infrastructure investment weak and the US ramping up tariffs on Chinese exports."
Ambitious targets to reduce pollution could be aided by new research out this month, showing that sulfur dioxide is not the only cause of the capital's pollution woes.
A joint study by US and Chinese scientists found that a large amount of sulfur in smog is created as the result of a chemical reaction between formaldehyde and the sulfur dioxide released by burning coal.
Reducing the amount of formaldehyde -- which comes from gas stove and kerosene heater emissions, and cigarette smoke, among other sources -- could therefore have an outsized effect on the overall amount of air pollution compared to simply targeting coal emissions.
"By including this overlooked chemistry in air quality models, we can explain why the number of wintertime extremely polluted days in Beijing did not improve between 2013 and January 2017 despite major success in reducing sulfur dioxide," said study lead author Jonathan Moch.
"Policies aimed at reducing formaldehyde emissions may be much more effective at reducing extreme wintertime haze than policies aimed at reducing only sulfur dioxide ... Our research points towards ways that can more quickly clean up air pollution. It could help save millions of lives and guide billions of dollars of investment in air pollution reductions."