The Level of Dioxins in Air in Hong Kong

The Level of Dioxins in Air in Hong Kong

(16 January 2020)

The Dioxins Monitoring Results

There are no major dioxins emission sources in Hong Kong. The trace amount of dioxins comes mainly from different types of combustion processes including vehicle engines, incineration facilities, occasional fires and hill fires. Therefore, the level of dioxins in the air has been very low.


The Environmental Protection Department (EPD) has been monitoring the dioxins levels at Central/Western and Tsuen Wan for many years. The Dioxins in Air report for December has been uploaded to the EPD webpage. The monitoring results show that the dioxins levels in Hong Kong are very low and have been decreasing for the past two decades. The annual average in 2019 is about 0.02 pg/m3 and is close to the lower detection limit. (See Fig. 1)

Figure 1: The Annual Trend of Dioxins 1998 - 2019

Fig. 1 The Annual Trend of Dioxins 1998 - 2019

Fig. 2 and Fig. 3 illustrate the 24-hr average levels of dioxins in the past 5 years. They are much lower than the Japan annual standard and the Canada 24-hr standard. As the prevailing wind direction differs in winter and summer, the dioxins concentration levels also vary with the seasons. The levels in winter are higher than that in summer. Dioxins have a long life time. If there were large amount of dioxins produced during the social events in the past few months, either from the actions of the demonstrators or from tear gas projectiles, the background levels as monitored by EPD would have increased significantly. However, the monitoring data obtained by EPD in the past few months up till December does not show any abnormal increases. This indicates that the amount of dioxins in the air have not been increased substantially by the social events in the past few months.

Figure 2: The 24-hr Average Dioxins Concentrations Measured at Central/Western Monitoring Station 2015 – 2019

Fig. 2 The 24-hr Average Dioxins Concentrations Measured at Central/Western Monitoring Station 2015 – 2019

Figure 3: The 24-hr Average Dioxins Concentrations Measured at Tsuen Wan Monitoring Station 2015 – 2019

Fig. 3 The 24-hr Average Dioxins Concentrations Measured at Tsuen Wan Monitoring Station 2015 – 2019

Dioxins Produced by Burning Process

According to international literatures (Ref. 1-4), dioxins would be formed during combustion as long as the material has a small amount of chloride such as PVC or salt. In Hong Kong, PVC is commonly found in electric wirings, cables, plastic pipes, plastic floor tiles, auto parts, road plastic fences and traffic cones. There are also folding tables, folding chairs and office chairs which are made of plastic. Since Hong Kong is an offshore city, sea salt is extensively present in our environment. Therefore, burning waste in the open air can indeed produce a small amount of dioxins. The Chinese University of Hong Kong (CUHK) has collected soil samples containing black smoky substance near a charred mattress. The analysis results show that the levels of dioxins concentrations are significantly higher than the background levels as found in the other samples, and consider that this higher levels may be caused by burning plastic or the plastic part of the mattress. This matches with the findings in international literatures. However, the dioxins concentrations found do not pose any health risks.

Would Tear Gas Produces Dioxins

One of the main components of tear gas is CS, which contains chlorine molecules. In theory, it is possible that a small amount of dioxins would be produced when it is burned at a high temperature, just like other chlorine-containing materials. However, when a tear gas projectile is launched, the duration of it staying at a high temperature is very short. Therefore, the amount of dioxins produced is much less than that produced by open burning of other materials. There are several international studies on the chemical substances released from CS at various temperatures, but dioxins were not found in any of the research reports (Ref. 5-10).


The CUHK and The Hong Kong Polytechnic University have collected air, water and soil samples at locations where large numbers of tear gas projectiles were deployed as well as other various locations within the campus. The Hong Kong Science and Technology Parks Corporation has also conducted a similar exercise at Hong Kong Science Park. So far, the dioxins levels measured are within the safety standard values. This indicates that the tear gas did not increase the dioxins levels in these venues which matches with the findings in international literatures.




Major sources and emission factors of dioxins

  1. Mengmei Zhang, Alfons Buekens & Xiaodong Li (2017) “Open burning as a source of dioxins.” Critical Reviews in Environmental Science and Technology, 47:8, 543-620, DOI: 10.1080/10643389.2017.1320154.
  2. UNEP (2013) “Toolkit for Identification and Quantification of Releases of Dioxins, Furans and Other Unintentional POPs under Article 5 of the Stockholm Convention.” January 2013.
  3. Takayuki Shibamoto, Akio Yasuhara, and Takeo Katami (2007) “Dioxin Formation from Waste Incineration.” Reviews of Environmental Contamination and Toxicology. 190:1–41.
  4. Schleicher O., Jensen A.A., Blinksbjerg P., Thomsen E., Schilling B. (2002) “Dioxin emissions from biomass fired energy plants and other sources in Denmark.” Organohalogen Compounds 56 (2002), pp. 147-150. (Emissions from BBQ)


Study on thermal decomposition of tear gas material

  1. Kluchinsky, A.J., P.B. Savage, M.V. Sheely, R.J. Thomas, and P.A. Smith. (2001) “Identification of CS-derived compounds formed during heat dispersion of CS riot control agent.” J. Microcolumn Sep. 13:186–190.
  2. Smith, P.A., T.A. Kluchinsky, P.B. Savage, et al. (2002) “Traditional sampling with laboratory analysis and solid phase microextraction sampling with field gas chromatography/mass spectrometry by military industrial hygienists.” Am. Ind. Hyg. Assoc. J. 63:284–292.
  3. Kluchinsky, A.J., M.V. Sheely, P.B. Savage, and P.A. Smith (2002) “Formation of 2-chlorobenzlidenemalononitrile (CS riot control agent) thermal degradation products at elevated temperatures.” J. Chromatogr. 952:205– 213.
  4. Timothy A. Kluchinsky , Paul B. Savage , Robert Fitz & Philip A. Smith (2002) Liberation of Hydrogen Cyanide and Hydrogen Chloride During High-Temperature Dispersion of CS Riot Control Agent, AIHA Journal, 63:4, 493-496, DOI: 10.1080/15428110208984739.
  5. Joseph J. Hout , Gary L. Hook , Peter T. LaPuma & Duvel W. White. (2010) “Identification of Compounds Formed During Low Temperature Thermal Dispersion of Encapsulated o-Chlorobenzylidene Malononitrile (CS Riot Control Agent).” Journal of Occupational and Environmental Hygiene. 7:6, 352-357, DOI: 10.1080/15459621003732721.
  6. Xue, Tian, Zhao, Qi-zhi, Han Yong-he, Lyu, Ning. (2015) “Thermal Decomposition of CS by TG/DSC-FITR and PY-GC/MS.” International Conference on Mechatronics, Electronic, Industrial and Control Engineering (MEIC 2015).
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Last revision date:22:07 18-01-2020