| Q1. |
What
are dioxins? |
| A1.
|
Dioxins are
environmentally persistent organic pollutants (POPs) comprising
polychlorinated dibenzo-para-dioxins (PCDDs) and polychlorinated
dibenzofurans (PCDFs). PCDDs and PCDFs are unintentionally formed
and released from thermal processes involving organic matters and
chlorine as a result of incomplete combustion or chemical reactions.
Dioxins are also produced by natural processes, such as volcanic
eruptions and forest fires. |
| |
Since dioxin-like
polychlorinated biphenyls (PCBs) and dioxins are families of structurally
related compounds and have similar properties, international efforts
have been made in addressing both dioxins and dioxin-like PCBs.
PCBs were manufactured for approximately 50 years as components
of insulating fluids inside transformers and other electrical
equipment.
|
| Q2. |
Do
all dioxin compounds pose the same level of toxicity? |
| A2.
|
Dioxins
are most often found in mixtures rather than as single compounds
in the environment. More than 400 types of dioxin-related compounds
have been identified and about 30 of these are considered to have
significant toxicity, with 2,3,7,8-tetrachloro-dibenzo-para-dioxin
(TCDD) and 1,2,3,7,8-PeCDD being the most toxic forms of dioxins.
Scientists use a shorthand method for comparing the toxicity of
different types or mixtures of dioxins to the toxicity of 2,3,7,8-TCDD
and 1,2,3,7,8-PeCDD. This method is called the "Toxicity Equivalence"
or TEQ.
|
| Q3. |
What
are the major sources of dioxin contamination? Where can we find
them? |
| A3.
|
Dioxins are
not produced on purpose. They are by-products of industrial processes
but can also result from natural processes, such as volcanic eruption
and forest fires. The incineration of waste containing certain types
of plastics (e.g. polyvinyl chloride) or similar materials with
incomplete combustion, the manufacture and use of certain herbicides,
chlorine bleaching of pulp and paper are the major sources of releases
of dioxins to air and water. Dioxins are also found in vehicle exhaust
and cigarette smoke. The US Environmental Protection Agency considers
"uncontrolled combustion", including open burning of household
waste and agricultural burning, to be the largest unaddressed sources
of dioxins in the environment. A recent study conducted by the Danish
Environmental Protection Agency indicates that meat grilling by
charcoal also constitutes a major source of dioxins emission. |
| |
Dioxins
are found throughout the world in practically all media, including
air, soil, water, sediment, and food, especially dairy products,
meat, fish and shellfish. The highest levels of these compounds
are found in some soils, sediments and animals. Very low levels
are found in water and air.
Since dioxins
occur naturally in the environment, it can never be totally eliminated.
|
| Q4. |
What
happen to dioxins when they enter the environment? |
| A4. |
Dioxins
may be transported over long distances when released into the
air; therefore, they are found in most places in the world. When
released into water, dioxins tend to accumulate in the sediments
where they can be further transported or ingested by fish and
other aquatic organisms. Since dioxins are broken down in the
environment very slowly, they can be deposited on plants and taken
up by animals and aquatic organisms. When ingested by animals,
dioxins tend to accumulate in fat. Dioxins tend to bio-accumulate
in the food chain and animals have higher concentrations of dioxins
than plants, water, soil or sediments.
|
| Q5. |
What
are the exposure routes of dioxins and the health effects of dioxin
exposures? |
| A5.
|
According to
the United Nations
Environment Programme's (UNEP) document "Dioxin and Furan
Inventories" published in May 1999, the consensus views on
the exposure routes of dioxins are as follows: 'Scientists today
agree that the major pathway of human exposure to PCDD/PCDF accounting
for >95% of the human intake is via ingestion of food. Uptake of
dioxins through water and soil (toddlers), inhalation, and dermal
contact are of minor concern.' |
| |
The report
"Compilation of EU Dioxin Exposure and Health Data" published
for the European Commission Directorate General-Environment and
the former UK Department of the Environment, Transport and the Regions
in October 1999 reported that 'The most important route for human
exposure to dioxins is food consumption, contributing 95-98% of
total exposure.' |
| |
The World
Health Organisation's fact sheet "Dioxins and Their Effects
on Human Health (June 1999)" says that 'It is estimated that
90% of human exposure to dioxins is through the food supply.' |
| |
The European
Union (EU) has not set a standard for ambient air since the EU experts
consider that inhalation is a very insignificant route of exposure
to dioxins. |
| |
According to
the UNEP's "Information on Dioxins", dioxin exposures
to humans are associated with increased risk of severe skin lesions
(chloracne and hyperpigmentation), altered liver function and lipid
metabolism, general weakness due to drastic weight loss, depression
of the immune system, and endocrine and nervous system abnormalities. |
| |
However,
it should be noted that the potential health effects associated
with dioxins, as with any chemicals, are directly related to the
level of exposure : the lower the exposure, the less the likelihood
of adverse effects. While dioxins have the potential to produce
a broad spectrum of adverse effects in humans based on information
from animal studies, there is no scientific consensus that dioxins
cause adverse health effects in people at today¡¦s low levels of
ambient dioxin concentrations.
|
| Q6. |
What
is the level of human exposure to dioxins through inhalation in
Hong Kong? How does it compare with other countries or international
standards? |
A6.
|
According to
the findings of the
study "An Assessment of Dioxin Emissions in Hong Kong"
completed in March 2000, the total inventory in 1997, i.e. adding
up the emissions from all known potential sources, was estimated
to be between 23 to 33 g I-TEQ with 21 to 27 g I-TEQ attributed
to the old municipal solid waste incineration plants. It was far
less than comparable inventories elsewhere in the world. As the
municipal solid waste incineration in old incineration systems contributed
to most of the emissions of dioxins, the level of dioxin emissions
has been significantly reduced after the old incineration plants
at Lai Chi Kok, Kennedy Town and Kwai Chung had stopped operation
in 1990, 1993 and 1997 respectively. |
| |
EPD monitors
ambient dioxins at two stations in the territory, one at Tsuen Wan
and the other at Central/Western. Average concentration over the past seven years (1998 to 2004) ranged from 0.051 pg/m3 to 0.120 pg/m3. These
levels are lower than or comparable to levels in large cities
in the world. |
| |
It has also
been reported that the concentration of dioxins in cigarette smoke
(around 1.81 ng I-TEQ/m3, which is 18 times higher
than the emission standard of 0.1 ng I-TEQ/m3 for modern
incinerators) would have more significant impact on human health
because cigarette smoke is inhaled directly into the lungs without
diffusion and/or dilution.
|
| Q7. |
Is
there any accredited laboratory in Hong Kong capable of doing analysis
of dioxin samples? |
| A7.
|
There are
two accredited laboratories, namely the Government Laboratory
and the laboratory at the Chemical Waste Treatment Centre, which
can analyse the relevant congeners of dioxins down to picogram
level from samples of stack gas, ambient air, waste water, sediment
or vegetation.
|
| Q8. |
Concentration
and toxic level of dioxins are reported in the units of TEF, I-TEF,
TEQ, and I-TEQ. What does each unit represent? |
| A8. |
Of the seventeen
PCDD and PCDF congeners with chlorine in the 2, 3, 7 and 8 positions,
2,3,7,8-TCDD is the most toxic, and by convention is assigned a
toxicity rating of 1.0, called a Toxicity Equivalent Factor (TEF).
The remaining 2,3,7,8-positional congeners are then assigned lower
TEFs relative to the toxicity of 2,3,7,8-TCDD. |
| |
The toxicity
of any mixture of PCDDs and PCDFs, relative to 2,3,7,8-TCDD, can
then be expressed by multiplying the concentrations of the 2,3,7,8-positional
congeners present in the mixture by their respective TEFs. The resulting
products for each congener are called Toxicity Equivalents (TEQs),
with units identical to that in which the concentrations of the
individual congeners are expressed. The TEQ of the mixture is obtained
by summing the individual TEQs. |
| |
In 1988,
the North Atlantic Treaty Organisation's Committee on the Challenges
of Modern Society (NATO/CCMS) developed a toxicity rating system
for dioxins under which the TEFs are termed International TEFs,
or I-TEFs. All congeners other than the seventeen 2,3,7,8-positional
congeners that may be present in a sample are assigned a TEF value
of 0.0. The summation of individual TEQs for a mixture of PCDDs
and PCDFs is termed the International Toxicity Equivalent or I-TEQ
of the mixture.
|
| Q9. |
How
does the EPD monitor the level of dioxins in Hong Kong environment? |
| A9. |
The EPD has
a comprehensive monitoring programme for dioxin emissions. Details
are summarized below: |
| |
| (a) |
Environmental
monitoring programme: |
| |
-
Ambient air monitoring
EPD monitors ambient dioxins at two stations in the territory,
one at Tsuen Wan and the other at Central/Western. Average concentration over the past seven years (1998 to 2004) ranged from 0.051 pg/m3 to 0.120 pg/m3. These
levels are lower than or comparable to levels in large
cities in the world.
|
| |
|
| |
- Marine
sediments and biota
EPD collected marine sediment from 15 monitoring stations for dioxins analysis in 2003. The samples were tested for 7 PCDD congeners and 10 PCDF congeners. The dioxin/furan contents in marine sediment ranged from 6.3 to 31.5 I-TEQ pg/g. Higher levels were generally found in typhoon shelters than in open marine waters. Three of the most toxic dioxin congeners, namely 2,3,7,8-TCDD, 1,2,3,7,8-PeCDD and 2,3,4,7,8-PeCDF were not detected at any of the monitoring stations.
For local marine biota, samples of intertidal mussel Perna viridis were also collected at five locations of Hong Kong shoreline in 2003 for the analysis of tissue concentration of dioxins. Of the 17 dioxin congeners analysed, only 3 were commonly found in the mussel tissues, i.e. OCDD, 1,2,3,4,6,7,8 ¡V HpCDD and 2,3,7,8-TCDF. Other dioxin congeners analysed (including the most toxic ones) were all below detection limits in the mussel samples.
|
| (b) |
EPD's Study to evaluate the emission of dioxins in Hong Kong |
| |
EPD has collected soil samples from the vicinity of old emission sources: animal cremator, hospital incinerators, and village incinerators for dioxin analysis.
|
|
| Q10. |
Is
the Chemical Waste Treatment Centre which is equipped with an incinerator
emitting dioxins to a level which endangers members of the public? |
| A10. |
It is very
unlikely that the operation of the high temperature incinerator
at the Chemical
Waste Treatment Centre (CWTC) on Tsing Yi Island will release
dioxins at a level that endangers human health. The CWTC has adopted
the best practicable technology to control emission of dioxins
to meet the stringent dioxin emission standard of 0.1 ng I-TEQ/m3
(i.e. 100 pg I-TEQ/m3 ). Apart from monitoring of dioxin
emissions at the stack, dioxins in the ambient air are monitored
by the operator of the Centre every month at the Cheung Ching
Estate, which is in the vicinity of the CWTC. Since 1999, the annual average concentration of ambient dioxins level is in the range of 0.044 pg/m3 to 0.077pg/m3. The readings are comparable to those in the ambient air measured
at the other two air monitoring stations at Central/Western and
Tsuen Wan, and are lower than or comparable to levels observed
in other large cities in the world. Moreover, there is no scientific
consensus that dioxins cause adverse health effects in people
at today's low levels of ambient dioxin concentrations.
The results
of the dioxin
measurements are available at the EPD's website : [http://www.epd.gov.hk/epd/english/environmentinhk/waste/data/data_cwtc.html]
|
| Q11. |
Will
Hong Kong build incinerators to deal with municipal solid waste
bearing in mind incinerators could be a key source of dioxins? |
| A11. |
In addressing
the problem of increasing quantity of municipal solid waste (MSW),
the Government¡¦s priority action is to promote waste reduction,
reuse and recovery. However, even with the best effort, we still
have a substantial quantity of solid waste which could not be recycled
and would require further treatment and disposal. Since the three
strategic landfills which currently receive the solid waste will
soon be filled up, in anticipation of the shortage of disposal capacity,
we need to consider other treatment
technologies for bulk waste reduction. |
| |
The Government
has an open mind on whether modern incinerator or other technologies
should be adopted for bulk waste reduction. We consider that the
technologies need to meet the highest international environmental
standards, be cost-effective, reliable and well proven. In April
2002, the EPD invited local and international companies to express
interest and to submit proposals for the processing and disposal
of MSW. After the close of the invitation in July 2002, we received
59 submissions and we are now in the process of evaluating them
before selecting the appropriate technologies for Hong Kong. The Government will consult the public before making a decision on the waste treatment technologies to be adopted. |
