HUMAN HEALTH AND THE
ENVIRONMENTAL IMPACTS OF USING SEWAGE SLUDGE ON FORESTRY AND FOR
RESTORATION OF DERELICT LAND Task 1 –
Desk-based literature review of the human health impacts of spreading
sewage sludge on non-agricultural land UKLQ09
August 2008
Use
of the report The technical report has
been developed through a collaborative project, managed and facilitated
by SNIFFER and has involved the members and partners. It provides
background information, within the confines of the project brief, to
support and inform member organisations and others. Whilst the document is
considered to represent the best available scientific information and
expert opinion available to the consultant at the stage of completion
of the report, within the confines of the specification given, it does
not represent the final or policy positions of SNIFFER or any of its
partner agencies, and it recognises that the historic practices
regarding sewage sludge recycling discussed are not current practice
within the UK.
Background to research
Although there is a great deal of research and scientific data on
sewage sludge application to land, much of this relates to modest
application rates on agricultural land. In recent years there
has been a substantial increase in the amount applied to forestry, and
to former opencast coal sites in the UK for purposes of land
restoration. Application rates of sewage sludge have been
considerably higher than traditionally practiced, and sewage sludge has
been applied using different techniques. There is concern
that poorly managed practices could result in risks to human health,
water, air and soil quality and biodiversity. Public and
political interest is high and this project will address the urgent
need to review this activity and develop decision support systems and
guidelines to ensure that the activities will not affect public health
or adversely affect the environment. The report outlines the
findings of the detailed desk-based literature review carried out
relating to potential health impacts.
Objectives of research
In detail, the project aims to:
To carry out a detailed desk-based literature review;
Follow this up with quantitative assessment of sites where
sewage sludge has already been applied;
Use this information to develop a systematic site
suitability and risk assessment procedure.
The objective of the report is to fulfil the first aim in relation to
the human health effects of spreading sewage sludge on non-agricultural
land. However, the literature on the health status of sewage exposed
populations proved to be small and the majority of studies are of
occupationally exposed populations. Consequently the original
approach to this task has been limited and only broad estimates of risk
have been achieved at this first stage.
Key findings and
recommendations
Contents of sewage
There is a variable literature on the content of sewage
sludge which might inform on potential health effects. The
units of measurement vary across studies making direct comparisons
difficult. In addition, some comes from abroad which may not
be directly usable in the UK setting. However, some broad
comments can be made.
Untreated sewage sludge is capable of harbouring sometimes
high levels of a wide range of bacteria (including drug-resistant
forms), viruses and parasites and while differing forms of sewage
treatment reduces levels of each of these, the effect is of varying
degree depending on factors such as initial concentrations and
resistance of the organism to treatment.
Chief amongst these organisms, with respect to risk to
humans, are Salmonella
spp, E. coli, Campylobacter, Giardia and
Ascaris. Hepatitis viruses can be found in
sewage as do a
range of other viruses if looked for, and it is likely that intensive
searching for specific organism may often result in positive finds even
if at low levels which may not be important in terms of risk to health.
Sewage sludge is a significant source of cadmium and lead
and to a lesser extent mercury and arsenic. Many chemicals,
including a wide range of pharmaceutical agents, can be found in sewage
at variable concentrations. Treatment of sewage can result in
dilution or concentration of these substances depending on their
chemical interactions and properties. It is difficult to
identify a “best treatment” which would reduce
chemical content across the board.
While radioactivity can be detected in sewage sludge (from
a range of sources including medical treatment and from industrial
sources) levels are likely to be low although no repeated data on
content in sewage sludge could be identified in the time available for
this project.
Treatment processes reduce the level of pathogenic
micro-organisms (including drug-resistant strains) and PAHs in sewage
sludge only partially, and the pathogenic organisms that survive
treatment are often of human origin. However, natural
attenuation in soil of organisms from applied sewage sludge can occur
although to a variable extent where different factors (e.g. site) will
affect persistence. Clear identification of surviving
organisms derived from non-human animals has not been clearly defined.
Exposure
A key step in understanding
and evaluating the risks posed by the application of sewage sludge to
land involves creating a framework to identify the exposure
compartments and transfer processes that have the potential to bring
about contact between humans and the hazardous materials present in the
sewage sludge.
We have created a basic
compartmental model to describe the potential for exposure among three
population groups: those occupationally exposed during the application
process; the local population that live close to the sewage application
area; and recreational users of the land area at a time-point after
application. Although there are many problems in
extrapolating from occupational to bystander exposures, because of the
lack of literature in the latter situation, it is both necessary and
inevitable that the occupational setting be considered in this report
especially as most of the health literature on sewage sludge is from
occupationally exposed populations.
The three main exposure
pathways are by inhalation, ingestion and trans-dermally. Of
these, in the context of sewage sludge to populations exposed
non-occupationally, potentially the most important is the ingested
route either directly, from water run-off or from food grown on sewage
treated soil. These are theoretical concepts because there
are no data from sewage application to land which would allow
estimation of risks.
Occupational
exposures
Many studies in
this area,
for both occupationally and non-occupationally exposed populations,
suffer from
small sample sizes which is predictable as many studies were undertaken
in response to a real or perceived problem in a defined
population. This limits the ability of such studies to
identify small levels of risk in the exposed populations and thus runs
into the problem of believing that effects do not occur whereas they do
but at low level. The only way this can be dealt with is by
larger studies which might need to incorporate multiple populations
with similar exposures, with adequate measures of exposure.
With these caveats some broad conclusions can be made on the literature
available but it is not possible to identify sufficient information to
derive quantitative estimates of risk.
Sewage workers experience
a wide range
of exposure to airborne endotoxin, a pro-inflammatory molecule produced
from bacterial degradation. Flu-like illness and lower
respiratory tract symptoms are associated with endotoxin exposure,
sometimes with high odds ratios, one study showing a dose response
relationship between exposure and symptoms.
The literature on the
prevalence and
incidence of symptoms in sewage treatment plant workers suggests a wide
variety of health complaints, with significant odds ratios in the range
of 2.2 to 9.4 which provides strong evidence of an effect on health in
these workers. The commonest are gastrointestinal symptoms,
diseases of the airways and skin problems. However, methods
of data collection for the presence of symptoms varies, notably in
terms of time frame (e.g. ever/never or during a certain time before
the investigation). There is no clear evidence of effects on
lung function.
In some studies no effects on any health outcome were
detected which
might represent a true effect, perhaps due to specific work practices
or differing relevant exposures, but more likely is due to reporting
bias with subjects underreporting symptoms for fear of loss of
employment especially if they had not been blinded to the objective of
the study. This is a reasonable conclusion given the high
odds ratios found in other studies.
Studies on infections and
infestations
in association with exposure to sewage show that workers exposed to raw
sewage are at most risk. The risk of infection is largely
dependent on the prevalence of infections in the populations served by
the treatment plants and on the treatment processes employed. Hepatitis
A and giardiasis are the best recognised infections, but the
association between hepatitis B markers and sewage exposure suggests
that waste water workers should be vaccinated against both hepatitis A
and B.
The limited evidence
suggests there is
potential for the development of cancer among sewage exposed workers,
in particular urothelial tumours and primary liver cancer.
The findings of increased levels of urinary mutagens among sewage
workers provides some exposure based support for this conclusion.
There are no data
sufficient which
enables quantitative risk assessment of health risks from occupational
exposure to sewage (with no studies on sewage applicators) and, in
addition, no data which would allow assessment of the presence or
otherwise of thresholds of effect for specific agents. There
are no studies where measured exposures were compared to measured
outcomes where both are numerically quantified.
Non-occupational exposures
Non-occupational
exposures, either as
residents or as by-standers, have been studied in a limited way and
mostly in countries very different from Scotland and Northern Ireland
(e.g. China, Kuwait, India). This literature suggests risks for HEV
infection and perhaps Salmonellae
and inhaled viruses but with more
clear associations with Giardia
and worm
infestations. The literature does, however, contain
one formal trial of exposure (Dorn, et al.1985) which showed no
increases in markers of ill health in the exposed population although
the odds ratios were generally positive. This suggests that
if there is an effect, larger studies would need to be conducted
although the content of the sewage sludge, poor application approaches
and high application rates could result in a more clear cut
risk. There is no strong evidence for chemicals causing ill
effects in these populations.
In the majority of
situations the
infective dose of a pathogen cannot be stated as a precise number of
pathogens guaranteed to cause disease in the host, although in some
outbreaks of E. coli
O157 as few as 10 to 100 organisms caused
disease. However, higher exposures may not always result in
disease. Resistance or immune responses to any pathogen vary
from person to person depending on, for example, the current health of
the person or their antibody levels to the pathogen under consideration
i.e. susceptible individuals or those with compromised health. Further,
strain to strain variability in pathogenicity exists.
Overall, this limited
literature is
conflicting but shows no consistent effect on health from living near
sewage treated land. The Dorn study is the most rigorous and,
being negative, generally supports this view. However, to address the
issue more completely would require substantial research investment,
requiring better formulated studies involving multiple sites over time
to capture both short and long term effects. A shorter term
study to capture the risk of infections and infestations would bring
results within about three years but would likely be very costly.
There is theoretically a
risk from
exposure to foods grown on land subjected to sewage sludge application
particularly from those foods eaten raw with no chance of cooking
destroying bacterial or other pathogen content. However,
again, the literature gives no clear estimate of likely risk other than
in the broadest terms and much of this relies on knowledge of what
infections might result from known exposures in other
settings. However, some pathogens can be taken up by plants
through their roots (e.g. salad crops eaten uncooked) and this might be
of wider concern than health impacts from local populations if shown to
be a real risk for the UK setting.
Odour
Odour is the main
complaint of
populations non-occupationally exposed to sewage sludge.
Ambient odours may produce health effects by direct irritation and/or
toxicological routes, through responses which genetically coded or
learned aversions or may be due to co-pollutant exposure(s) that is
part of the odorant mixture. It is also generally accepted
that unpleasant odours act as warning signs or as indicators of
potential risk to health but not as direct triggers to ill health.
The nose is a very
sensitive organ
being able to detect very low concentrations of some aromatic
compounds. The fact that a smell can be detected does not
necessarily mean that the cause of that smell is adverse in terms of
inducing an organic response in an individual. But where
there is a perception that the source of the smell is potentially
toxic, the fact that a smell is detected is often taken by the public
as proof that the cause of the odour is truly toxic.
Separating out these perceived effects from genuine effects can be
impossible.
With respect to sewage
sludge, four
main groups of compounds contribute to the characteristic odours:
sulphurous compounds, nitrogenous compounds, acids and aldehydes and
ketones. Some of these at high concentrations have the
potential to cause health effects but there is no scientific literature
which has specifically considered the effects of odour on health from
exposure to sewage sludge. It is likely that such odours will
affect the quality of life to those regularly albeit intermittently
exposed, although regular exposure may result in tolerance and loss of
recognition of the odour. In some cases this can be
potentially dangerous as in occupational exposure to hydrogen sulphide
where loss of appreciation of the presence of the gas can lead to
increasing exposure. In parallel, workers in smelly
occupations regard this tolerance effect as a benefit as they continue
to work in what others would regard an impossible environment,
providing a continuing income. As this aspect of sewage
sludge seems to be most important to those exposed, studies in this
area might be worthwhile although perceptions and pre-conceptions do
make such studies difficult to design. Such studies should
involve expertise in odour research.
Risk estimation of
non-occupational exposure to sewage
sludge
Because the literature is
so sparse
and in particular the lack of studies which allow definition of dose
response relationships for any potential exposure it has not been
possible to use a source pathway receptor approach to the information
available. However, it should be appreciated that the time
scale for this project was very short and it is likely that there is
other literature which may prove helpful in specific areas.
However, we are able to
rank risks
from non-occupational exposure to sewage sludge in the broadest terms
as follows:
Category
Risk
Infections:
Salmonella,
Giardia, E coli, Hepatitis A, B and
E
Moderate
to low
Infestations
Low
Chemicals
(inc pharmaceuticals)
Low
Metals
Low
Radioactivity
Very
low
Table: Broad
qualitative estimates of risk to health from exposure to
sewage sludge treated land
The authors have therefore come to the
conclusion that there are potential risks to human health that may
arise from the treatment of fields with sewage sludge or from its run
off but in many situations effects are poorly understood
mechanistically. However, from the information currently available the
authors are not in a position to quantify these risks.
In the case of exposure to chemicals,
effects such as reproductive outcomes are theoretical problems with no
human epidemiological data to support this, although as these have not
formally been looked for this cannot be regarded as an absence of an
effect. Effects from infections such as enterobacteria are
more clear (especially perhaps Salmonellae) but again how big the issue
is as a contributor to the overall burden of these infections across
Scotland cannot be defined.
The authors believe, therefore that
two options are open at this stage: either to accept there is a risk
and attempt to find ways to reduce exposure (the precautionary
approach) or to collect more specific information and define more
clearly the true risks. Both approaches together might also
be appropriate. Defining more clearly the risk will be
expensive and difficult but some suggestions for ways of improving the
evidence base are summarised in the following research recommendations.
There is an absence of data on airborne
soil and water run-off levels of microbes, chemicals and metals from
sewage treated land in the UK. To some extent the second
phase of this study may address this but will not provide information
on conventional use of sewage sludge to land as the target sties are
those where sewage is being used as land fill. Provision of
such data is essential in determining quantitative risk
assessment. In particular:
What is the level of a range
of marker pathogens (bacteria, viruses, protozoa and helminths) within
soil treated with sewage and in run-off and how long do they survive?
What are the levels of key
marker chemicals and metals in sewage sludge and in run-off from
treated land?
What environmental
conditions post application (e.g. temperature, rainfall, wind speed)
might affect pathogen survival and chemical concentrations in the
run-off? We anticipate that much of this information could be generated
by laboratory based experiments or modelling.
Knowledge of soil-based and run-off
contaminant concentrations would then allow some estimation of likely
exposures to the range of individuals exposed to sewage sludge. It is
important to know:
How much chemical/metal gets
into fetal and adult human tissues?
How do these levels relate
to levels in animal models where
environmental exposure to
sewage sludge cause developmental defects?
What mechanisms are involved
in human uptake and any subsequent effects, especially during sensitive
windows e.g. during in-utero development?
What are the risks of
exposures to mixtures of chemicals and metals? (This is an
area for basic scientific research but is unlikely to produce results
quickly enough to inform on any immediate proposed regulatory moves,
however.)
There may be some risk to health
(specifically from infections) to populations living near land to which
sewage sludge has been applied but from our examination of the
scientific literature it is not possible to quantify the degree of
risk. The best paper in this area suggests no such risk but
exposure situations will vary from place to place and it is our feeling
that such risks are both plausible and possible. To define
this risk studies would be needed of populations living near sewage
application sites. These will be logistically difficult and
expensive especially if hoping to define some of the less likely risks
to more unusual health end-points. Such studies would need to
investigate multiple communities as each one alone would not provide
sufficient statistical power.
Identification and recording of use of
sewage sludge application in Scotland, with indications of how well
adherence to application guidelines was followed would allow study of
health effects in the local communities in relation to exposure to such
land. This is logistically difficult at the record keeping
level but is relatively easy to do from the health point of view using
Geographic Information Systems (GIS) processes, provided that records
of target conditions such as episodes of infectious diarrhoea
(especially Salmonella) or pneumonia can be drawn from routinely
collected data.
An alternative approach would be to
study occupationally exposed workers and from the exposure-response
data gathered, extrapolate the degree of risk to those
non-occupationally exposed populations who are less exposed.
This is the approach taken by EPAQS, but is not without its
difficulties as a range of assumptions have to be made.
However, such an approach would also provide HSE with health and
exposure data for this particular work force for which limited UK
information is currently available. However, it should also be
recognised that workers exposed to low levels of pathogens may build up
immunity leading to under estimates in the health of the general
population.
The most useful way of addressing these
issues is to take an integrated approach to the various aspects of
emission, exposure and effect data. We believe that
identifying key target agents and key potential health outcomes as
markers of these interactions would enable decisions to be made on
approaches to reducing exposures to levels deemed appropriate, on the
assumption that if there are risks of lesser degree from other agents,
these will automatically fall in line with those which exert a greater
effect. This approach is implicit in the approach to air
quality control.
In addition to this we would recommend
that further research is carried out to identify those factors that
influence the perceptions of a local community with respect to the
health risks associated with sewage sludge application. Other studies
have shown that the acceptability of any given risk is often influenced
by the likelihood and degree of the risk; the reversibility of the
health effect; the knowledge or familiarity of the community with the
health effect; whether the risk is voluntarily accepted or
involuntarily imposed; whether the community is compensated for their
exposure to the risk; the advantages of the policy or activity; and the
risks and advantages of any alternative action. It is essential that
those involved in policy relating to sewage sludge application have as
full an understanding of these areas as possible and engage in clear
risk communication to the communities most affected.
Key words: sewage sludge, health, respiratory disease, infections,
cancer
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