Central Kowloon Route in Hong Kong Opened, Equipped with the World's Largest Tunnel Air Purification System

December 22, 2025 | Golden Chen

The Hong Kong Central Kowloon Bypass, with an investment of HK$42.36 billion, officially opened to traffic on December 21, 2025. The passage utilizes the world's largest tunnel air purification system. This system was designed, manufactured, and installed by Clean Tunnel Air International AS., a subsidiary of the AirQuality Group.

Kowloon Bypass, also known as the Central Kowloon Tunnel, connects Yau Ma Tei in the Yau Tsim Mong District with Cha Kwo Ling in the Kwun Tong District, forming a new east-west underground passage. The passage is 4.7 kilometers long, including a 3.9-kilometer tunnel section. The entire route features a three-lane dual carriageway design and is expected to significantly alleviate traffic congestion on major arterial roads in central Kowloon. According to the Hong Kong Highways Department's forecast, after the tunnel opens, travel time through central Kowloon during peak hours will be reduced to approximately 5 minutes, saving about 25 to 30 minutes compared to existing routes, thus significantly reducing traffic pressure on existing arterial roads.

Given that the Central Kowloon Bypass is primarily an underground passageway and serves as a vital alternative transportation route through the Kowloon Central District, its traffic volume will remain at a high level for an extended period after its operation. Therefore, from the initial planning and construction phases, the project has set far higher requirements for air quality control in and around the tunnel.

Long tunnels accumulate large amounts of vehicle exhaust fumes. The main pollutants in vehicle exhaust include particulate matter, carbon monoxide (CO), hydrocarbons (HC), nitrogen oxides (NOx), which pose significant risks to the human respiratory and cardiovascular systems. If these fumes are directly released from the tunnel entrance, they will threaten the health of nearby residents. The construction of urban tunnel entrances often sparks protests from local residents and concerns from environmentalists. To alleviate these concerns and ensure public health and safety, the Central Kowloon Bypass has constructed three large ventilation towers, located in Yau Ma Tei, Ho Man Tin, and the Kai Tak Development Area . Each tower is equipped with three advanced air purification systems to efficiently remove pollutants before they are released into the atmosphere from the tunnel exhaust .

To achieve the project's stringent environmental design goals, the Hong Kong Highways Department conducted a global tender for suppliers and ultimately selected Clean Tunnel Air International AS (CTA) as its suppliers , responsible for the specialized design, technology development, and implementation of the air purification system.

To address this need, CTA 's air purification system employs an "active purification" technology approach, significantly different from traditional tunnel ventilation methods that rely on air dilution and natural diffusion. This system integrates high-efficiency electrostatic filtration, harmful gas removal, and automatic cleaning technologies , and is equipped with a real-time intelligent monitoring system. This system dynamically adjusts operating parameters based on changes in traffic flow and fluctuations in pollutant concentrations, ensuring continuous and stable operation in complex urban environments and high-traffic scenarios. The entire system comprises nine air purification stations, handling an air volume of up to 2250 m3/s, equivalent to 8.1 million m3/h, making it the world largest air purification system.

In terms of practical effectiveness, the application of this system can not only directly optimize the air environment inside the tunnel and reduce the concentration of pollutants that pedestrians and vehicles come into contact with, but also reduce the pollution level of the purified air emitted to the ground through the treatment of exhaust gas, thereby helping to mitigate the impact of traffic activities on air pollution in the surrounding areas and providing technical support for environmental governance in high-density urban areas.

CTA dedicated five years to this project, completing the overall system design, multiple rounds of testing, technical optimization, and final on-site implementation . Team members from Shanghai, Hong Kong, Norway, and other parts of the world worked day and night to complete the project on time and to a high standard. To ensure the design and technical reliability of the air purification system, CTA built a 100,000 m3/h physical testing platform at its Shanghai factory, capable of testing pollutant treatment efficiency under varying temperature and humidity conditions. Only after extensive testing and verification did the team begin actual installation at the Hong Kong site to ensure a successful installation test on the first attempt.

General Manager Johan Gjengstø stated: “The Central Kowloon Bypass is not only an important transportation project, but also a crucial environmental project. Our tunnel purification team has spent five years dedicated to creating a reliable, efficient, and high-quality air purification system for the large tunnels. We will also provide operation, maintenance, and quality assurance services to ensure the system's reliable and stable operation. ”

The tunnel air purification system is expected to bring significant environmental and social benefits. Improved air quality inside and around the tunnel will help reduce health risks associated with air pollution, especially in the densely populated urban area of Kowloon, which will be of great significance to residents and commuters. Urban planners and environmental experts believe that the project demonstrates how modern infrastructure can improve travel efficiency while also taking into account ecological environment and public health responsibilities.

Clean Tunnel Air International AS (CTA), from Norway, was founded in 1989 and focuses on improving air quality in tunnels, implementing numerous tunnel air treatment systems worldwide. In 2020, CTA merged with AirQuality Group, and through technology and market integration, provides customized solutions for global transportation infrastructure and large-scale urban environmental protection projects.