Environmental Engineering Reference
In-Depth Information
Table 21.4
Dose-response relationships
that about 10-15% of AIDS patients die of com-
plications related to cryptosporidiosis. However,
examination of UK mortality data (ONS 2007)
indicates a mean age of death from cryptosporid-
iosis of 63.7 years, and of death from HIV disease
of 42 years. Over the period 2001 to 2005, in the
UK, there were limited (6) deaths from crypto-
sporidiosis and only one incident of mortality
under 50 years of age. With such limited data, it
is inappropriate to try to quantify mortality from
cryptosporidiosis in AIDS cases separately from
that of the rest of the population. In this QMRA,
the influence of HIV/AIDS was restricted to an
estimation of the additional severity and duration
of the infection in the immunocompromised
population.
Beta-Poisson
Pathogen
Exponential Reference
a
b
r
Campylobacter
spp.
0.145
7.584
Medema et al.
1996
Cryptosporidium
parvum
0.004005
Teunis et al.
1996
Rotavirus
0.265
0.442
Haas et al.
1993
Exposure assessment
The demographic profile of the flooded population
was outlined above (see 'Flood scenario'). It was
necessary tomake a number of assumptions about
exposure during the clean-up process in order to
conduct the QMRA.
Viruses
Infection is assumed to proceed to clinical illness
in 50% of cases (WHO 2004). The severity weight
of 0.093 is based on an uncomplicated case of
diarrhoea (VGDHS 1999). The duration of symp-
toms is usually between 3 and 8 days (Ruisin
et al. 2000; HPA 2007b). Rotavirus infection is not
usually fatal in people over the age of 4 years
(ONS 2007), thus fatalities were only estimated
for children under the age of 5 years.
Clean-Up
In this example it was assumed that the majority
of the residents would wait until the flood water
had dropped considerably, before returning to the
flooded property to begin the remediation process,
and would continue to live elsewhere until the
property was restored. The duration of the clean-
up process will depend upon the size of the prop-
erty, the number of people involved and the extent
of the flood damage. The initial clean-up process
(when people may be exposed to pathogens) was
assumed to last between 1 and 4 days. Daily flood
water contact, during this period, was assumed to
be the length of the working day (up to 14 hours,
with a mean daily exposure of 7 hours), with those
involved returning to alternative accommodation
at the end of the day. It was assumed that, where
possible, children would be kept away from the
clean-up, with children under the age of 5 not
present during the cleaning process. A normal
distribution was assumed to account for the pro-
portion of older children present, with an average
of 10% presence for children aged 5 to 9 and 20%
for children aged 10 to 14. It was assumed that all
household residents (aged 15 and over) would be
involved in the clean-up, but no accountwas taken
Dose-Response assessment
For microbial hazards the dose-response charac-
terizes the relationship between exposure and
the incidence of the health effects, as exposure to
the hazard does not necessarilymean that a health
impact is inevitable. In many cases, depending on
what dose is received (and how), the body may be
able to remove the pathogen without any obvious
ill effect. Dose-response relationships are typical-
ly characterized by exposing a population of
volunteers (usually young healthy people) to var-
ious concentrations of the microorganism under
investigation. The results are then modelled (gen-
erally using exponential and beta-Poisson models
- for further information see Fewtrell et al. 2008b)
to enable extrapolation to low doses. The dose-
response parameters shown in Table 21.4 were
used in the QMRA.
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