PTWI

The Joint FAO/WHO Expert Committee determined that PTWI for mercury via all possible physiological routes should not exceed 5 pg per g of body weight, and only two thirds may be in the form of a methyl derivative (WHO, 1993). 

All food products contain some lead. They usually do not exceed the level of 0.1 to 0.2 pg per g, although venison may contain up to several pg per g due to its contamination via ammunition. In order to lower lead intake in Poland, the highest allowed concentrations have been established for various food products, within a range from 0.2 pg per g (for milk) to 2 pg per g (dried fungi). For most products, the values are set between 0.1 and 0.3 pg per g (Dz. U., 2003). The Joint FAO/WHO Expert Committee calculated that lead PTWI should not exceed 25 pg per g of body weight (WHO, 1993). It is particularly important that infants and children should be protected against the possibility of lead uptake. 

At present, Polish standards only limit zinc content in juices and nectars. The maximum limit for zinc in these products is 5 pg per g (Dz. U., 2003). The Joint FAO/WHO Expert Committee recommends the PTWI should not exceed 7000 pg per g (7 mg per g) of body weight (WHO, 1989). 

At present, Polish standards limit the content of tin only in food products packed in tin-coated containers, and in fruit and vegetable preserves, and products including such preserves, packed in other materials (Dz. U., 2003). The content of tin in products intended for children up to the age of three must not exceed 10 pg per g, and for other products it must not exceed 100 pg per g (for food in tin-coated packing) or 20 pg per g (for food in other types of packing). The Joint FAO/WHO Expert Committee established the PTWI value for tin as 14,000 pg per g (14 mg per g) of body weight (WHO, 1989). 

Alarmingly, higher uptake of lead and cadmium (when compared to PTWI) has been observed in children, especially in industrial areas, and in... 

The term acceptable is used widely to describe safe levels of intake and is apphed for chemicals to be used in food production such as, e.g., food additives, pesticides, and veterinary dmgs. The term tolerable is applied for chemicals unavoidably present in a media such as contaminants in, e.g., drinking water and food. The term PTWI (Provisional Tolerable Weekly Intake) is generally used for contaminants that may accumulate in the body, and the weekly designation is used to stress the importance of limiting intake over a period of time for such substances. The tolerable intake is similar in definition and intent to terms such as Reference Dose and Reference Concentration (RfD/RfC), which are widely used by, e.g., the US-EPA. For some substances, notably pesticides, the ARID (Acute Reference Dose), is also established, often from shorter-term studies than those that would support the ADI. The ARfD is defined as the amount of a substance in food that can be consumed in the course of a day or at a single meal with no adverse effects. 

Do the same principles apply to contaminants that have TDI or PTWI values ... 

It was recommended that excursions above the TDI/PTWI should be considered in the way as for substances where an ADI has been set. It was emphasized that contaminants having very long half-lives accumulate in the body and the chronic toxicity is most often manifested when critical concentrations are achieved in target tissues. Furthermore, there are usually large differences between the acute or shorter-term toxic doses and the chronic LOAELs. In such cases, peak excursions of several times the PTWIs for short periods (days, weeks, or even months) or lower peak intakes for even longer periods (months to years) may be of no consequence provided that the integrated exposure over longer periods does not lead to critical steady-state tissue concentrations. 

For many substances the body s own mechanisms for de-toxification and repair mean that low doses of some chemicals can be tolerated without experiencing any adverse effects. However, once a certain threshold has been exceeded then the degree of adverse effect is related to the dose. The highest dose at which no adverse effects are observed in the most susceptible animal species is identified at the No Observed Adverse Effect Level (NOAEL). The NOAEL is used as the basis for setting human safety standards for contaminants, Provisional Tolerable Weekly Intakes (PTWIs) or Tolerable Daily Intakes (TDIs).1... 

The PTWI is defined as the amount that an individual can ingest weekly over a lifetime without appreciable health risk. It is related to the NOEAL so that ... 

Uncertainty factors usually have a default value of 100 so that the PTWI is usually equal to the NOEAL x 700. If human data are available then UFi is usually taken to be one. 

Intakes that exceed the PTWI will not necessarily result in any adverse effect because the uncertainty factors are designed to be conservative. In practice it is... 

Non-thresholded chemicals that are not carcinogens are less frequently identified. For many years lead was considered to be thresholded because its effects on haemoglobin synthesis were not seen at low doses. However, recent work into the effects of lead on mental development suggest that there may be no threshold for this end-point. Food is a relatively minor source of lead exposure compared with air and dust in urban environments. For chemicals that relate to toxicological end-points that do not show thresholds it is not possible to identify a NOAEL or PTWI. In such cases it is desirable to estimate the level of risk associated with a given level of exposure. 

In recent years it has become increasingly apparent that for chemical contaminants that are abundant in the environment a more sophisticated approach to dose-response characterisation is required. There is increasing evidence that small but significant sub-populations are exposed to intakes that exceed PTWIs and most people are exposed to potential carcinogens through their diet. In such cases the PTWI concept is redundant because it is necessary to assess the actual levels of risk to which individuals are exposed in order to introduce proportionate control measures. Simply knowing that the hazard exists is not sufficient. 

Tables 2.4 and 2.5 provide estimates of intake based on the average of the lead concentrations in Table 2.1 and kidney consumption distributions summarised in Tables 2.2 and 2.3. Intake figures tend to follow food consumption patterns although for pork kidney the intake is zero because no lead was detected in it. As expected, when calculated on a per person per week basis children have lower intakes of lead than do adults. However, when consumption is corrected for individual bodyweight, as is necessary for comparison with a PTWI, children s intake of all lead is about three times that of...

Risk evaluation is an apparently simple task of comparing an estimate of intake with the PTWI. If intakes are below the PTWI then there is no risk whereas if they exceed the PTWI then some risk management action may be required. For non-thresholded contaminants risk is assumed to be proportional to intake and therefore intakes should be as low as practically achievable. In practice risk evaluation is a far less certain science. 

A vital and often over-looked aspect of risk evaluation is ensuring that the estimate of intake corresponds to the PTWI so that like is being compared with like. For example, toxicological end-points are frequently time-related. On rare... 

The outputs from risk assessment will normally include information about the relationship between dose and risk and estimates of levels of doses and thus risks in the population. For contaminants that have a toxicological threshold the Provisional Tolerable Weekly Intake (PTWI) might be defined and the number of consumers who have the potential to exceed this level of intake quantified. If a PTWI cannot be established (such as for genotoxic carcinogens) then it may be possible to quantify the proportion of a population exposed to a given level of risk by using QRA methods. If QRA methods cannot be applied then a qualitative assessment can be made such as to reduce intake levels to as low as is reasonably practicable. In either case it is the function of risk management to identify an optimal course of action to minimise the risk to consumers. 

In many cases the risk assessment might indicate that the proportion of consumers with the potential to exceed the PTWI is zero and that there is no need for any action to be taken to control risks. However, even in these circumstances other risk management activities, such as risk communication, might be appropriate. 

If there is the potential for consumers to exceed the PTWI then it will be necessary to consider measures to control risks. The simplest approach would be to identify the level of contamination in a foodstuff that would cause a high-level consumer to exceed the PTWI and to introduce legislation or some other form of control to establish that as the maximum permitted concentration. In practice there may be multiple routes of potential exposure and so all potentially affected foods should be taken into account. Setting maximum permitted concentrations for all affected foods would then require that the PTWI be apportioned between foods according to the potential intake from each food. This method assumes that a high-level consumer always consumes foods that contain the contaminant at the maximum permitted level. In reality this is an... 

Aggregate exposure assessment is naturally more complex than the methods used for dietary risk assessment. In the simplest analysis a worst case can be established for each source and exposure route and then summed to give a total exposure. If this were below any threshold of concern such as the PTWI then no further action would be required. However, if the total worst case exposure was above a PTWI then it is unlikely to reflect the real situation since the probability that any individual would be exposed to each source by each route at the maximum level is very remote. 

The risk to health from chemicals in food can be assessed by comparing estimates of dietary exposure with recommended safe levels of exposure. For most metals and other elements, these are the Provisional Tolerable Weekly Intakes (PTWIs) and the Provisional Tolerable Daily Intakes (PTDIs) recommended by the Joint Expert Committee on Food Additives of the Food and Agricultural Organisation of the United Nations and the World Health Organisation International Programme on Chemical Safety (JECFA). The European Commission s Scientific Committee on Food has established other relevant safe levels. These are Acceptable Daily Intakes (ADIs) for chemicals added to food, and Tolerable Daily Intakes (TDIs) for chemical contaminants. The use of the term tolerable implies permissibility rather than acceptability. All the above recommendations are estimates of the amount of substance that can be ingested over a lifetime without appreciable risk, expressed on a daily or weekly basis as appropriate. 

The exposure to lead in food by the general population in the UK is well within international tolerable limits. Results from the TDS indicate that during the period 1976 to 1997 the dietary exposures over the whole population fell from 0.11 to 0.026 mg per person/day (Table 7.1).6 This excludes any contribution from drinking water, which is likely to be higher in areas with soft water. The PTWI recommended by JECFA is 0.025 mg/kg bodyweight, equivalent to 0.21 mg/day for a 60 kg adult. The dietary exposures for mean and 97.5th percentile consumers in 1997 were 0.024 mg/day and 0.043 mg/day respectively, well below the PTWI. These dietary exposures to lead in the UK are similar to those in Canada (0.024 mg/day),7 The Netherlands (0.01 to 0.032 mg/day),8 and the USA (0.015 mg/day).9... 

Duplicate diet studies of people living in areas where exposure to lead from other sources is known to be low have provided some valuable information. The mean dietary lead exposure to women was estimated to be 0.31 mg/week and the mean lead exposure to children as 0.11 mg/week.12 These studies included the contribution from drinking water. In areas with elevated levels of lead in tap water, estimated lead intakes of both adults and children are found to be higher and, in a small percentage of cases, above the PTWI. 

However, the results of duplicate diet studies have shown that there are localised high intakes by consumers in certain areas or by consumers of certain foods. In the old mining village of Shipham in Somerset, UK, where the cadmium levels in some vegetable samples were more than 1 mg/kg, the dietary exposure to cadmium of the study population was about double the average weekly dietary intake and some individuals exceeded the PTWI for cadmium.19... 

Investigations into the presence of mercury in food have been carried out in a comparatively small number of foods in the UK since 1966. Mercury is included in the analytes measured in the samples of the Total Diet Study. The estimated dietary exposures to total mercury (organic and inorganic) for the general population as determined from the UK TDS (Table 7.5) have remained fairly constant between 0.002 and 0.005 mg/day. The dietary exposures of mean and 97.5th percentile consumers in the UK in 1997 were 0.0031 mg/day and 0.0064mg/day.10 These may be compared with the JECFA PTWI for mercury of 0.005 mg/kg bodyweight/week (of which no more than two-thirds should be methyl mercury),31 which is equivalent to 0.043 mg/day for a 60 kg adult. The dietary exposures are similar to those in the USA (0.008 mg/day)8 and the Netherlands (0.002 mg/day)7 but lower than New Zealand (0.013 mg/day).18... 

The JECFA PTWI for tin is 14 mg/kg body weight, equivalent to 120 mg/day for a 60 kg adult. Average dietary exposure has remained in the range 1.7-5.3 mg/day (Table 7.6). The dietary exposure estimates for mean and 97.5th percentile consumers were 1.9 mg/day and 6.3 mg/day respectively, well below the PTWI. 

Aluminium has been included in the UK TDS only since 1988, owing to the difficulty of obtaining reliable analytical data in the presence of environmental levels of aluminium (Table 7.7). Dietary exposures for mean and 97.5th percentile for UK consumers in 1997 were 3.2 mg/day and 5.7 mg/day respectively,10 similar to the population exposure. All the estimated exposures were well below the JECFA PTWI of 7 mg/kg bodyweight, which is equivalent to 60 mg/day for a 60 kg adult. The population estimate is similar to dietary exposures reported for other countries. A dietary exposure in the USA of 11.5 mg/day was reported for 14-16-year old males,8 while dietary exposures in an Italian TDS were between 2.3-6.3 mg/day.37... 

From the 1997 UK TDS, the mean and 97.5th percentile consumer dietary exposures were 0.1 mg/day and 0.17 mg/day respectively. There is no JECFA PTWI or PTDI for chromium. 

Cadmium may accumulate in the body, primarily in kidneys and liver, and has a half-life of several decades. The toxic effect occurs in the kidneys and may lead to proteinuria. A PTWI value has been established at 7 p,g kg-1 body weight equivalent to 72 p,g person-1 day-1 [9]. Cadmium has been classified as a... 

The intakes are given as the mean and 95th percentile values or, for Se, as the mean and 5th percentile values. The PTWI and the Nordic Nutrition Recommendations (NNR) indicate upper tolerable and target intakes, respectively. For Ni neither recommended intake nor PTWI value exists. 

The assumption is made that less than 5% of the total content in seafood is inorganic arsenic [16] for which a PTWI value has been set. 

For children of 4-6 years of age the mean and 95th percentile intake of cadmium (Table 9.5) at 36 and 57 percent of the PTWI, respectively, are relatively high. The margin is modest between the young child s intake of Cd via food in Denmark and the PTWI value that corresponds to the adverse effect level among the most sensitive individuals. When setting the PTWI value an average... 

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