A recent study has found that school classrooms are more polluted than the outdoors.
Scientists in Australia and Germany insist some school classrooms may contain higher levels of airborne ultrafine particles.
The experts fear children easily inhale these particles deep into the lungs.
Lidia Morawska with her team studied levels of ultrafine particles in 3 elementary school classrooms in Brisbane, Australia.
They found that on numerous occasions ultrafine particle levels in the classrooms were significantly higher than outdoors.
The highest levels occurred during art activities such as gluing, painting and drawing when indoor levels were several times higher than outdoor levels.
There also were significant increases in ultrafine particle levels when detergents were used for cleaning.
Evidence suggests that they can be toxic when inhaled into the lungs.