76. an average adult breathes about 8.50 x 103 l of air per day. the concentration of lead in highly polluted urban air is 7.0 x 10 g of lead per one m' of air. assume that 75% of the lead is present as particles less than 1.0 x 10 m in diameter, and that 50 % of the particles below that size are retained in the lungs. calculate the mass of lead absorbed in this manner in 1 year by an average adult living in this environment