TS.4. Estimation of the 131I Activities Ingested by People
Once the time-integrated 131I concentrations in fresh cows' milk produced in any county of the U.S. have been estimated from equation TS.1, it is necessary to determine how much milk was produced, how much was left available for fluid use, what was the delay between the production of milk and its consumption by people, and where it was consumed in order to derive the time-integrated 131I concentrations in the milk consumed by people and the corresponding 131I intakes. Accordingly, information is needed on the milk production in each county, on the milk distribution pattern within each county and each state, on the delay between production and consumption of milk, and on the consumption of milk as a function of factors such as age and sex.
The production of milk in a given county, i, in the 1950s was estimated from county data on the number of cows, C(i), published by the U.S. Department of Commerce in the 1954 Census of Agriculture combined with state statistics on the average annual milk production per cow, CP(s), in the state, s, published by the U.S. Department of Agriculture. Assuming that the average milk production per cow reported for the state, s, did not vary significantly from the average milk production rate in any county, CP(i), in the same state, the total annual production of milk in a given county, MP(i), was estimated as:
The total amount of milk produced in county i that is available for fluid
use, TMFU(i), is estimated by subtracting the amount of milk used on farms
for feeding calves and butter production, MUF(i), and the milk used to manufacture
dairy products, MM(i), from the total amount of milk produced, MP(i):
The amount of milk that is used on farms for feeding calves and for butter
production in county i, MUF(i) (referred to as "milk
used on farms" in this report), is estimated by apportioning
the state value for the milk used on farms, MUF(s), as reported by USDA
according to the number of cows in each county:
The total amount of milk used in each state for the manufacture of dairy
products, MM(s), is reported by the USDA but data for each county are not
available. Since milk for fluid use would have brought a higher price, it
can be assumed that only the surplus, after the fluid needs of the population
of that county had been met, would have been sold, at a lower price, to
manufacturing plants. To estimate the milk used for manufacture of dairy
products in each county, it is assumed that in counties where more milk
was produced than was needed for fluid use in that county, a portion of
the milk produced was purchased by a manufacturing plant (located in that
county or near-by). The amount of milk used in each county with a milk surplus
for the manufacture of dairy products is estimated using the value for the
state, MM(s), and apportioning it between the surplus counties, for which
the total milk production is denoted as TMP(s), according to the total amount
of milk produced in that county, MP(i):
Estimates of annual volumes of milk available for fluid use in each county of the contiguous U.S. in the 1950s are provided in the report.
TS.4.3. Milk Distribution
The volume of milk available for fluid use estimated by this model is either consumed on the farm, distributed for consumption to the local county population, or distributed to areas outside the county where the amount of available milk does not meet the consumption needs of the population. The distribution of milk to other counties usually results in the mixing of milk from a number of sources that may have varying 131I concentrations as a result of differences in fallout deposition.
Milk, in general, was produced close to the population centers that required the milk supply, but the increasing use of refrigerated tank cars and the reduced cost of transportation also made it possible to ship milk to greater distances to satisfy major urban areas and to fulfill emergency shortages.
To simulate the flow of milk, neighboring counties have been grouped together into "milk regions". The geographic extent of the milk regions is based on the Crop Reporting Regions and milkshed areas outlined by each state's Department of Agriculture. Additional milk regions were drawn to isolate the population concentrated around cities in each state. For the states close to the NTS (Nevada, Arizona, Utah and part of California), available information on milk distribution and pasture practices were used to designate boundaries of the milk regions. A total of 429 milk regions have been defined in the contiguous U.S.
Information on volumes and directions of milk distribution and on the delay times between production and consumption is, in general, more qualitative than quantitative. Although relevant data have been published for federally administered Milk Marketing Orders and for parts of the west, they do not provide all of the information required in this study and cannot be used to derive values for the entire country. It was therefore decided to resort to a simple model based on the nationwide statistics on milk production and utilization reported by the U.S. Department of Commerce and the U.S. Department of Agriculture, and to validate as much as possible the structure of the model and the assumptions used by means of published information and recollections of experts. Given the uncertainties included in the assessment, it was deemed sufficient to derive only one model of milk distribution for the 1950s and to use the 1954 data for that purpose.
In this model, the total milk for fluid use in the county, TMFU(i), is divided into four categories corresponding to the consumption of milk by the following population groups:
category 1: those living on the farms of the county where the milk was produced;
category 2: those living in the county where the milk was produced but not on farms;
category 3: those living in a group of neighboring counties within a designated "milk region
category 4: those living at greater distances, that is, in other milk regions" in the same or another state.
The model assumes that the milk produced in a county was used initially to satisfy the consumption needs within that county and, if there was a surplus, to fulfill the needs that had not been satisfied elsewhere.
The consumption of fluid milk of category 1 in a given county, MFC(i), was assumed to occur with a delay of one day following its production by the cow and to be proportional to the number of farms in that county, FA(i):
The volume and source of milk in category 2, milk consumed in-county but not on farms, is dependent on the amount of milk available in the county. The expected milk consumption in the county, EC(i), was subtracted from the total volume of milk for fluid use available in the county, TMFU(i); the result indicates whether the balance of milk in the county, MB(i), was surplus or deficit:
According to whether the value of MB(i) is positive or negative, milk is exported to, or imported from, other counties. Milk in category 2 is assigned a delay time of 2 d between production and consumption.
The first step used in the model to balance the surplus (or deficit) of milk in an individual county is by flow of milk between counties in the same "milk region". The volume of milk pooled from the counties with a surplus of milk is distributed to the counties of the milk region with a deficit of milk, proportionate to their needs. This volume of milk, exported to deficit counties within the milk region, constitutes the milk of category 3, to which a delay time of 3 d is assigned.
If the volume of the surplus of milk in the milk region does not entirely cover its needs, additional milk must be provided by another milk region. Milk of category 4 is that which is imported into a deficit milk region from another surplus milk region or, conversely, that which is exported from a surplus milk region into a deficit milk region. It is assumed to have a delay time of 4 d between production and consumption because the milk in this category has traveled the furthest distance from producer to consumer. Movements of milk in category 4 between surplus milk regions and deficit milk regions were designed to achieve balance between production and consumption at the national level.
The assumptions regarding the direction and distance that milk was distributed during the 1950s are based upon Agricultural Research Stations reports as well as information made available from State Agricultural Department Milk Boards, Federal Milk Marketing Administrators Offices, and Agricultural Economists with the Extension Service. The overall surplus and deficits calculated for each region of the country, as a result of the needs of major population areas, drive the major patterns of milk flow. The fact that most of the surplus milk in the U.S. was produced in the northern part of the country and shipped south also has an important influence on the distribution patterns chosen.
Estimates of time-integrated concentrations of 131I in each category of fresh cows' milk have been obtained using this methodology for each county of the contiguous U.S. and for each of the 90 tests considered in the report. For each test, these estimates of time-integrated concentrations of 131I in each category of fresh cows' milk, as well as the uncertainties that are attached to those estimates, are presented in the Annex devoted to the test under consideration in the form of a Table. Figure TS.3 presents the time-integrated concentrations of 131I in volume-weighted milk summed over all tests.
The 131I intake from milk by man is the product of the time-integrated concentration of 131I in the milk ingested and of the milk consumption rate. Individual intakes of 131I from milk vary widely from person to person because of variability in such factors as environmental parameters, patterns of milk production and distribution, and dietary habits. Therefore, realistic estimates of individual intakes can be made only if specific information is available on the individual considered (age, sex, place of residence, source of milk, delay between production and consumption of milk, milk consumption rate). In the absence of personal data, only average intakes over large or homogeneous groups of people can be estimated with reasonable accuracy. For this reason, the 131I intakes of milk by man estimated in this report for each county and for each nuclear test are averages over specified population groups deemed to be representative of a large spectrum of individuals. However, all of the information necessary to estimate an individual thyroid dose is included in this report.
Although ingestion of cows' milk is generally the predominant contributor to the intake of 131I, other exposure routes need to be taken into consideration for individuals who consume little or no cows' milk. These exposure routes, which include inhalation and the ingestion of goats' milk, cottage cheese, leafy vegetables, and eggs, are considered in the report in a much less detailed manner than the ingestion of fresh cows' milk.
For each test, the estimates of time-integrated concentrations of 131I in ground-level air and in the foodstuffs that are considered, as well as the uncertainties that are attached to those estimates, are presented in the Annex devoted to the test under consideration in the form of a Table.