c="Canada";
a=CountryData[c, "Area"]
p=QuantityMagnitude[CountryData[c, "Population"]]
b=CountryData[c, "BoundaryLength"]
(b/(Sqrt[a p]))

(Quantity[37742157, "people", people, "People"])//FullForm

Exposure[c_]:=Module[{
a=CountryData[c, "Area"],
p=QuantityMagnitude[CountryData[c, "Population"]],
b=CountryData[c, "BoundaryLength"]
},
(b/(Sqrt[a p]))]

Sort[Exposure[#]->#& /@ Select[CountryData["Countries"], CountryData[#,"Population"]>Quantity[(10^6),"people"]&]]