116
C. Picot et al.
where E m is the individual exposure (mg/kg/day) to the phycotoxin, m, from the
ingested shellfish species, j, C mj is the concentration (mg/kg) of the same phycotoxin
in the edible portion of the same species, CR j is the daily consumption rate (kg/day)
of this species, P j is the proportion of a given shellfish species in a consumer diet
(unitless), and BW is the consumer body weight (kg) assumed, in this study, to be
60 kg (USEPA 2000).
The difference in acute- and chronic-exposure assessments stands in the consumption parameter to be used: the former takes into account the portion size of a
given shellfish species, whereas the latter considers the daily consumption rate of
all shellfish species.
In this study, focus was on five of the most consumed bivalve species in the
geographical area under study: oysters, mussels, cockles, carpet shell clams and
razor clams; king scallops were also considered in assessments about DA.
The approach used for exposure calculation usually depends on the nature of the
available data. In this study it was based on the probabilistic approach described in
Kroes et al. (2002).
Probabilistic Approach
Given that a shellfish consumer will not eat, at each time, the same portion size
and that the toxin level in the eaten portion will not be the same, the probabilistic
calculation considers all of the combinations of occurrence and consumption data.
Distributions for both the food consumption and the contamination data were used in
the model to simulate dietary intakes by repeatedly drawing random values for each
input distribution. The description of input variables in terms of distributions allows
one to characterise their variability and/or uncertainty. Monte Carlo simulation
techniques are used by the model to generate output distributions of dietary intakes
liable to be ultimately considered in probabilistic risk characterisation. Output
distributions (i) give several exposure data (mean, median, minimum, maximum
and all percentiles) and (ii) include a comprehensive analysis of the sensitivities
of the resulting exposure with respect to uncertainties in parameters (Counil et al.
2005; Kroes et al. 2002; Tressou et al. 2004). The @Risk package, version 4.5
(Palisade, USA) with the Microsoft Excel spreadsheet under XP (Microsoft, USA)
was used to perform risk analysis from Monte Carlo simulations and probability
distributions so as to develop the exposure model on taking into account uncertainty
and variability. Each simulation was run for 10,000 iterations to mimic the inherent
uncertainty in shellfish-contamination and -consumption as well as the uncertainty
in the mathematical process.
As the phycotoxin levels are affected by the cooking process in use (temperature,
dry cooking or not), this parameter has to be considered (McCarron and Hess 2006;
Vidal et al. 2009). The analyses were made on raw bivalves. To take into account the
cooking process impact, for each toxin, the ratio between the phycotoxin rates in raw
samples and in cooked samples was determined in a preliminary study. The latter
study was made using classical cooking conditions for an at home use of shellfish:
C. Picot et al.
where E m is the individual exposure (mg/kg/day) to the phycotoxin, m, from the
ingested shellfish species, j, C mj is the concentration (mg/kg) of the same phycotoxin
in the edible portion of the same species, CR j is the daily consumption rate (kg/day)
of this species, P j is the proportion of a given shellfish species in a consumer diet
(unitless), and BW is the consumer body weight (kg) assumed, in this study, to be
60 kg (USEPA 2000).
The difference in acute- and chronic-exposure assessments stands in the consumption parameter to be used: the former takes into account the portion size of a
given shellfish species, whereas the latter considers the daily consumption rate of
all shellfish species.
In this study, focus was on five of the most consumed bivalve species in the
geographical area under study: oysters, mussels, cockles, carpet shell clams and
razor clams; king scallops were also considered in assessments about DA.
The approach used for exposure calculation usually depends on the nature of the
available data. In this study it was based on the probabilistic approach described in
Kroes et al. (2002).
Probabilistic Approach
Given that a shellfish consumer will not eat, at each time, the same portion size
and that the toxin level in the eaten portion will not be the same, the probabilistic
calculation considers all of the combinations of occurrence and consumption data.
Distributions for both the food consumption and the contamination data were used in
the model to simulate dietary intakes by repeatedly drawing random values for each
input distribution. The description of input variables in terms of distributions allows
one to characterise their variability and/or uncertainty. Monte Carlo simulation
techniques are used by the model to generate output distributions of dietary intakes
liable to be ultimately considered in probabilistic risk characterisation. Output
distributions (i) give several exposure data (mean, median, minimum, maximum
and all percentiles) and (ii) include a comprehensive analysis of the sensitivities
of the resulting exposure with respect to uncertainties in parameters (Counil et al.
2005; Kroes et al. 2002; Tressou et al. 2004). The @Risk package, version 4.5
(Palisade, USA) with the Microsoft Excel spreadsheet under XP (Microsoft, USA)
was used to perform risk analysis from Monte Carlo simulations and probability
distributions so as to develop the exposure model on taking into account uncertainty
and variability. Each simulation was run for 10,000 iterations to mimic the inherent
uncertainty in shellfish-contamination and -consumption as well as the uncertainty
in the mathematical process.
As the phycotoxin levels are affected by the cooking process in use (temperature,
dry cooking or not), this parameter has to be considered (McCarron and Hess 2006;
Vidal et al. 2009). The analyses were made on raw bivalves. To take into account the
cooking process impact, for each toxin, the ratio between the phycotoxin rates in raw
samples and in cooked samples was determined in a preliminary study. The latter
study was made using classical cooking conditions for an at home use of shellfish: