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day’s intake, with a decay constant applied, as shown in Equation 1. This model can be configured to
simulate PFAS intake from breastfeeding or from formula feeding (or, with some modifications by the user,
both simultaneously.) For breastfeeding intake, daily PFAS concentrations in breastmilk were calculated
from the daily maternal serum concentration using a breastmilk transfer factor. For intakes from formula
feeding, the concentration of PFAS in formula was based on the PFAS drinking water concentration set by
the user.

Revisions to the model consisted of (1) improvements that relate to the model’s fidelity to available data,
including better tracking of the movement of chemical mass during intake, elimination, childbirth, and
breastfeeding; (2) updates of chemical-specific model parameter values; and (3) enhancements that improve
usability.

Model improvements
[9]
MDH’s original TK model was modified to more accurately account for the movement of chemical mass
and to better reflect real-world conditions.

Maternal loss from placental transfer
The infant serum concentration at delivery is calculated by multiplying the maternal steady-state serum
concentration by a placental transfer factor. The corresponding chemical mass, calculated as the product of
the infant’s serum concentration at delivery, the newborn’s body weight, and the volume of distribution
(V ), represents the mass of chemical in the newborn that is lost from the mother. To better account for the
d
maternal transfer and loss of chemical during gestation, the model was modified to subtract this transferred
mass from the mother’s body burden on the first day of the simulation period.

Milk intake phase-in and smoothing
[18]
Following delivery, a mother’s lactation rate takes time to ramp up to a stable value . In recognition of this,
milk consumption [Table 1] is phased in rather than abruptly started on the day of delivery. The updated
model permits the user to enter percentage values to be applied on the first four days of the model’s
simulation period. Milk volume increases in an approximately linear trend from 36 to 96 h postpartum, and
[18]
then levels off . Using the data from Neville et al. and considering the mean plus two standard deviations
to be a surrogate for the 95th percentile breastmilk intake rate, we calculated appropriate percentage values
[18]
that reflected the available data [Table 1].
Breastmilk intake rates were further refined by smoothing out the abrupt changes in intake that occur when
applying intake rates for four time periods between birth and one year of age. As was done with other
parameters, the intake rate for each interval was assigned to the midpoint of the interval, and intake rates for
all other days were calculated using linear interpolation.

Mass balance
In the original model, each day’s infant chemical intake was converted to a concentration using the current
day’s body weight and volume of distribution, while the previous day’s serum concentration was carried
over directly: