Findings from the Indoor Air Quality Study 2025

Photo: Left to right – Dr Ellis Hon (President of the Hong Kong Society of Paediatric Respirology and Allergy), Monica Chan (Chairperson of the Self-Help Development Centre, participant of the program), Prof. Zhi Ning (Professor, Institute of Environment, Division of Environment and Sustainability, the Hong Kong University of Science and Technology), Patrick Fung (CEO of CAN)

Program introduction

Clean Air Network conducted indoor air quality (IAQ) monitoring during the 10-month period from August 2024 to May 2025, with air quality monitors placed in 29 different premises for one month each to analyze the impact of different premises and human activities on indoor air quality. 

This small-scale study covers classrooms and halls of primary and secondary schools, kindergartens and child care centers, after-school care centers, church, shopping malls, restaurant, galleries, boxing gym, office, subdivided flats and transitional housing units. The monitoring devices mainly collect data on three major indoor air pollutants: carbon dioxide (CO2), particulate matter (PM), and volatile organic compounds (VOCs), as well as temperature and humidity.

Air quality is of concern in kindergartens, child care centers, transitional housing and subdivided flats

This study found that indoor air quality is usually poor in transitional housing and subdivided flats due to insufficient ventilation and high levels of particulate matter. In addition, places where children often stay, such as kindergartens, child care centers and after-school care centers; carbon dioxide concentrations often exceed the standard. School classrooms are also overcrowded, causing carbon dioxide to rise and accumulate. Prolonged exposure to indoor spaces with accumulated air pollutants may result in fatigue and allergic symptoms.

Dust storm and effects on indoor air quality

During the research period, there was a sandstorm in April. At that time, Hong Kong’s outdoor air quality deteriorated significantly, and the Air Quality Health Index (AQHI) reaching the highest level of 10+ (“serious”). At that time, four secondary schools participated in the one-month air monitoring, two of which were located in Kwun Tong, one in Tai Kok Tsui, and one in Tsuen Wan. On April 13 2024 (Sunday), the particulate matter level (daily average) exceeded the Hong Kong statutory air quality index by one to two times. This shows that the deterioration of outdoor air quality will also affect indoor air quality.

 

Case Studies

Case 1: The Dilemma of Opening Windows: Indoor Air Pollution in Transitional Housing

A transitional housing unit in Kowloon, with an area of ​​about 200 square feet, and home to a family of three. During the one-month monitoring period, significant problems were found in the indoor environment of this residence, especially the concentration of carbon dioxide and particulate matter, which may have adverse effects on the health of residents.

The carbon dioxide concentration within the flat exceeded the Environmental Protection Department standard 77% of the time and remained between 1,000-3,500 ppm for a long time, especially during weekday evenings and weekend noon. The highest record occurred on the morning of June 9, reaching between 2,000-3,500 ppm and lasting for 9 hours, with a maximum of 3,500 ppm. Being in a high carbon dioxide environment for a long time may cause residents to experience problems such as dizziness, fatigue and poor sleep quality, especially affecting young children and older family members.

In addition, the concentration of particulate matter also showed instability, often rising suddenly between 09:00 and 21:00, with the highest value exceeding 450 μg/m³, far exceeding the control target of 100 μg/m³. The increase in suspended particles is likely caused by environmental factors, such as construction sites and highways outside the window. Exposure to high concentrations of particulate matter in a short period of time may induce asthma and even increase the risk of bronchitis, which is particularly dangerous to the health of children and the elderly.

Based on the above data, the indoor air quality of this residential unit is poor, posing a potential threat to the health of family members. The necessity of improving air quality cannot be ignored, and effective measures must be taken to ensure the safety and health of the living environment.

Suggestion: The Window Dilemma

Poor air circulation often leads to accumulation of carbon dioxide, which affects health. The household owner said that he often closes the window due to dust, exhaust gas and noise outside the window. Therefore, when the window is closed, the fan can still be turned on to promote air circulation and evenly distribute carbon dioxide in the room. Secondly, turning on the exhaust fan can also effectively exhaust dirty air and further reduce the concentration of carbon dioxide, which is especially suitable for kitchens and toilets.

Case 2: Childcare centres to prevent carbon dioxide accumulation

An after-school care center of about 300 square feet in Sham Shui Po is managed by two tutors to take care of 6 to 8 children. A month-long indoor air quality monitoring showed that the carbon dioxide concentration was high for 52% of the time, ranging from 1,000 to 5,000 ppm, especially between 09:00 and 18:00. In multiple tests, the highest value recorded on the 14th, 27th and 29th was about 5,000 ppm. During the period, there were seven times when the carbon dioxide concentration exceeded 3,000 ppm for at least two consecutive hours. This value has exceeded the Good Class of the IAQ objectives by Environmental Protection Department, posing a potential threat to the health of children.

On the other hand, during the monitoring period, staff pointed out that the exhaust fans were often turned off due to their loudness, which was also the reason why carbon dioxide was difficult to dissipate.

During the monitoring period, a secondary school classroom in North District, a secondary school classroom in Kwun Tong, and another after-school care center in Kowloon also recorded the highest carbon dioxide concentration of 4000 ppm during class time, which lasted for 2-4 hours.

Solution:

Increase ventilation

If carbon dioxide accumulates during class, it means that the air circulation in the venue is not ideal. Replace the exhaust fan with a quieter one, or, if possible, slightly open the doors and windows to help bring fresh air into the classroom.

Reduce the number of people

Reducing the number of students in class or the time they spend together can help avoid shortening students’ stay in places with high carbon dioxide concentrations.

Case 3: Secondary school classrooms need ventilation

In a secondary school classroom located in Tsuen Wan, total volatile organic compound concentrations are often high 20% of the time, exceeding the EPD’s indicators, remaining at 600-1,000 μg/m³ during school hours. The highest value is recorded at noon of 26/3, approx. 1,200 μg / m³, other places are generally good, occasionally a little high.

The reason is likely related to whiteboards and markers. The use of blackboards and chalks brings obvious suspended particles. Many schools have switched to whiteboards and markers. If the air circulation in the classroom can be maintained, it should be able to effectively reduce total volatile organic compounds.

Three common misunderstandings and problems

  1. Air conditioner = air purifier?

Turning on the air conditioner will allow air to circulate. In fact, the air conditioner is not used to filter air, and the filter only filters dust, not air pollutants.

  1. The exhaust fan is too noisy so it is unused

Maintaining indoor air circulation can help disperse and reduce air pollutants. However, it is common that exhaust fans are not turned on because they make a lot of noise, so air pollutants accumulate and cannot be dispersed

  1. Don’t open the windows when you encounter a sandstorm

First, pay attention to whether indoor air is circulated with outdoor air during the sandstorm. If so, the outdoor dust may have already affected indoor air quality. When closing the windows, remember to turn on the air purifier or keep the indoor air circulating to reduce the accumulation of air pollutants.

Photo: Left to right – Dr Ellis Hon (President of the Hong Kong Society of Paediatric Respirology and Allergy), Monica Chan (Chairperson of the Self-Help Development Centre, participant of the program), Prof. Zhi Ning (Professor, Institute of Environment, Division of Environment and Sustainability, the Hong Kong University of Science and Technology), Patrick Fung (CEO of CAN)

Status of Indoor Air Quality Management in Hong Kong

At present, indoor air quality management in Hong Kong is mainly voluntary, relying on individuals and industry. However, due to the lack of attention, understanding and specific incentives, many people and industry players lack the motivation to improve indoor air quality, resulting in inadequate management in most areas.

Suggested Actions

Medium- and long-term measures

CAN recommends that schools and governments take the following measures to protect children’s health:

  1. Continuous monitoring of indoor air quality
    • Continuous monitoring of indoor air quality in child care centres, kindergartens, primary and secondary schools.
    • The government should encourage and subsidize schools to install indoor air quality monitoring instruments and take improvement measures, such as using air purifiers, when the concentration of suspended particles exceeds the standard.
  2. Extension of Government Guidelines to Child Care Centres
    • The current guidelines for student activities in kindergartens, primary schools and secondary schools are not applicable to child care centres. It is recommended that the Social Welfare Department formulate similar guidelines for child care centres.
  3. Continuously monitor air quality in all spaces
    • Indoor air quality varies with seasons and usage. Schools are advised to monitor the air quality of different spaces and make targeted improvements.
  4. Data transparency
    • Schools are advised to display monitoring data to students, staff and parents to raise awareness of air quality.
  5. Subsidize social service units
    • The Social Welfare Department should subsidize social service units to replace outdated air fresheners and ventilation equipment to prevent the recurrence of infectious diseases.
  6. Expanding monitoring scope
    • In addition to common air pollutants such as carbon dioxide and suspended particulates, TVOC should be included in the monitoring scope.

Summary

These measures aim to enhance the effectiveness of indoor air quality management in Hong Kong and protect children’s health.

References:

South Korea and Taiwan have relevant laws, showing the benefits of continuous monitoring of indoor air quality for students’ health

South Korea: The《School Health Act》 requires schools to install PM2.5 monitoring equipment in every classroom

Taiwan: Schools that have been notified on a batch-by-batch basis under the《Indoor Air Quality Management Act》 must install continuous monitoring facilities. The latest monitoring results should be immediately posted in the venue or at a conspicuous place at the entrance, and records should be kept.

For detailed statistics, please see the following file:

Indoor Air Quality Press release in English