If 10 micrograms of lead is present per deciliter of blood, the concentration in parts per billion (ppb) is

10 μg=10×10⁻⁶ g=10⁻⁵ g

1 deciliter=0.1 L=100 ml

Thus, 100 ml of blood contain lead = 10⁻⁵ g

$\therefore$1 billion (10⁹) parts will contain:
$=\frac{{10}^{-5}}{100}\times {10}^9=100 \mathrm{parts}$