The dust, smell, noise and heat from Hong Kong's many construction sites are often "unbearable" for many of the city's residents, particularly in its hot, muggy summer. "No matter if it's in winter or summer," said 28-year-old Dorothy Wong. "It's dusty and stinky from diesel. And so noisy that I usually have to cover my ears when I pass by them. In Hong Kong, it's not easy to go around them as the city is rather packed." Aside from being a nuisance for the city's residents, machinery and fuel consumption on construction sites also contributes to air pollution in the form of nitrogen oxides and particulate matter, which can damage the heart and lungs after prolonged exposure. Building and construction is a highly polluting sector, making up about 39% of the world's energy-related CO2 emissions. While there's a lack of data on how much construction sites contribute to the overall figure, one study of a building complex in Korea found that on-site construction contributed to 4.2% of total emissions. Part of the problem is the industry's reliance on carbon-emitting diesel generators to power energy-hungry machines like tower cranes. That's why in 2019 Hong Kong's major developer Gammon Construction started experimenting instead with lithium-ion batteries as a cleaner power source. Each Enertainer, as the battery systems produced by local startup Ampd Energy are called, can potentially replace two diesel generators that would power four tower cranes and slash on-site carbon emissions by over 80%. "A [diesel] generator in Hong Kong that powers a crane might produce around 140 tons of carbon a year," said Julian de Jonquieres, Ampd's chief operating officer. "But the electricity to charge our Enertainers would be around 25 tons." Gammon Construction has since used storage systems, housed in non-descript white metal boxes, to power heavy machinery at seven of its sites. And as urban populations and the demand for new buildings grow, the hunt is on for more ways to clean up construction sites, with cities at the forefront. In 2019, Norway's capital Oslo pioneered what it says was the world's first zero-emissions construction site when it turned a busy street into a pedestrian zone using mainly electric machinery. The switch from fossil fuel-powered equipment saved 35,000 liters (9,246 gallons) of diesel and 92,500 kilograms (203,930 pounds) of carbon emissions, according to local authorities. Oslo, alongside other metropoles such as Los Angeles, Budapest and Mexico City, has pledged to cut emissions by at least half for all new buildings and infrastructure projects by 2030, as well as using only zero-emissions construction machinery by 2025. The switch is slightly easier for Oslo compared to other cities, because it generates 98% of its electricity from renewable sources like hydropower. Still, finding fossil fuel free machinery and ensuring a reliable power supply for it has proved a challenge, according to Nils Gelting Andresen, who works with the city's climate agency. New tender criteria that encourage construction sites to go emissions-free, as well as city funding for machinery that runs on sustainable biofuels, electricity or district heating could help. But, says Gelting Andresen, there needs to be a global push for green construction sites to fuel demand for such equipment. "The most important work to ensure an increase in demand is international cooperation — as Oslo is a very small city — on a global scale," Andresen told DW. "In order to meet the high demand for on-site power, new technology and solutions will be necessary, and we already see these emerging now — like batteries and mobile charging solutions." One company trying to meet the so far limited demand for green machines is Wacker Neuson. Based in Germany, they've been producing electric construction equipment since 2015, including excavators, wheel loaders and battery-powered rammers. "Almost all of our zero-emission machines are battery-powered and can be operated independently of the power grid," according to the company. Between September 2020 and February 2021, the Danish capital Copenhagen cut carbon emissions from construction equipment by 85% on its first emissions-free site after switching to Wacker Neuson's electric machinery. But making the change is pricey. For instance, hydrogenated vegetable oil (HVO) diesel is 50% more expensive than traditional diesel, said Christina Schulin-Zeuthen, manager of the city's construction department." And the use of larger electrified machinery has shown to be more expensive," Schulin-Zeuthen told DW. "Hopefully, prices will decrease, as the market is moving towards climate-friendly solutions." Construction sites are just one source of emissions from the building sector. Creating cement, the world's main building material, is highly resource and energy-intensive, accounting for some 8% of global CO2 emissions. Operating buildings, that is powering lighting, heating and cooling them, also accounts for about 28% of global emissions. Albert Chan, a professor at the Hong Kong Polytechnic University's Department of Building and Real Estate, said cleaning up the construction phase of building is critical, but it can't stop there. "In the world of sustainability, the operation phase is deemed more significant and complex in ensuring long-term sustainability of buildings throughout their lifetime," said Chan. Still, Chan believes emissions-free construction sites could be a driving force in cleaning up the building sector and that sustainable machinery "will lead to a worldwide transformation." For residents of cities, such as Hong Kong, the change can't come soon enough. "It would be ideal for construction companies to reduce the extra heat and dust they release from the construction sites," said Wong. "But I know it's going to be quite difficult."