Scientists were unaware as to how commercial products containing these nanoparticles behave in real-life situations and were thus worried.
"You put these materials on your skin and they disappear," says researcher Professor Brian Gulson from Macquarie University's Graduate School of the Environment in Sydney.
"Surely they must go somewhere. Are they being absorbed through the skin?" he added.
Gulson and colleagues applied two coats of the sunscreen on two human volunteers within a gap of three hours and then took the blood and urine samples a few days afterwards.
"The data that we have so far indicates that every little is absorbed through the skin," says Gulson.
Another study led by Dr Paul Wright from the Royal Melbourne Institute of Technology examined the effect of these particles on immune system.
They tested the toxicity of a range of nanoparticles, including zinc oxide, to human immune cells over period of 24 hours.
They found that the zinc oxide nanoparticles were more toxic to the immune cells than nanosilver nanoparticles, diesel exhaust particles and silica dioxide particles only due to high dose of zinc oxide.
"We actually found that the zinc oxide particles were the most potent," said Dr Wright.
Though Gulson and Wright agree the results are reassuring but emphasise both their studies are limited.