Lead concentration in blood is considered alarming if it is:

Lead is the major heavy metal pollutant released by the vehicular exhausts. It persists in the air and precipitates with the rains. In reaches the human body when consumed through water or inhaled as suspended particle. At the concentration of (30 \mu $\mathrm{g}$/ 100 $\mathrm{ml}$) in blood, it is considered poisoning as it can cause aplastic anaemia, tumour and cancer. Hence, the correct answer is '(30 \mu $\mathrm{g}$/ 100 $\mathrm{ml}$)'.