Table 4.

External exposure space parameters for second-order polynomial fit to the simulated exposure space using a nonlinear least squares fit to Equation 4.

Derived BMD	Route of exposure	Species	${\mathrm{\beta }}_1$	${\mathrm{\beta }}_2$	${\mathrm{\beta }}_3$
${\text{BMD}}_{15}$	Oral	Rat	0.148	0.253	0.227
		Human	0.00848	0.0106	0.00787
	Inhalation	Rat	0.779	6.052	$-0.423$
		Human	1.334	3.386	$-2.151$
	Dermal	Rat	68.564	372.216	$-47.535$
		Human	0.288	1.619	$-0.249$
${\text{BMDL}}_{15}$	Oral	Rat	0.00241	0.0148	0.0262
		Human	$5.807\times {10}^{-4}$	$3.325\times {10}^{-5}$	$1.105\times {10}^{-3}$
	Inhalation	Rat	0.022	0.334	$-0.232$
		Human	0.0694	0.133	$-0.059$
	Dermal	Rat	4.299	18.99	2.718
		Human	0.069	0.133	$-0.059$

Note: Polynomial coefficients (${\mathrm{\beta }}_i$) are tabulated for each species, and route of exposure with corresponding dose–response curves are shown in Figure 6 (rat) and Figure 7 (human). Parameters for ${\text{BMD}}_{15}$ determine the total absorbed dose (TAD) versus fraction of day (FOD) exposed boundary resulting in a chlorpyrifos (CPF) brain concentration at the derived ${\text{BMD}}_{15}$. Similarly, parameters for ${\text{BMDL}}_{15}$ determine the boundary resulting in a brain CPF concentration at ${\text{BMDL}}_{15}$. Differences in physiologically based pharmacokinetic (PBPK) model parameters between rat and human simulations resulted in differences in the ${\text{BMD}}_{15}$ and ${\text{BMDL}}_{15}$ cutoff for each species. BMD, benchmark dose; ${\text{BMD}}_{15}$, benchmark dose at 15% cognitive deficit; ${\text{BMDL}}_{15}$, lower limit benchmark dose.