The standard for metal detectors is that they should be able to detect a .22 caliber bullet, which is just about 1mL in volume. If we then want to find out how much iron is required in a penis in order to trigger a metsl detector, we simply inject iron into the penis until it contains that same volume of metal, and since the average penis is about 170mL in volume, we need a concentration of 1/170 = 0.005882mL of iron per 1mL of penis. Since iron weighs 7.874g/mL, we need 0.005882 x 7.874g = 0.0463g of iron per mL in the penis.

A normal concentration of iron in blood is 0.5mg of iron per 1mL of blood, so we would need 46.3mg/0.5mg = 92.6 times the normal amount of iron in the blood in order for a metal detector to go off. That’s only 7.8g of iron, but you’d still die of blood poisoning.