Lead is a toxic metal that can cause serious health problems if ingested or inhaled. It is found in various sources, even after its addition to petrol has been banned.

Smelting units are one of the significant sources of lead exposure. During the smelting process, lead is released into the environment, which can be inhaled or ingested.

Paints, especially those used in older houses, can contain lead. When the paint peels or cracks, lead dust can be released into the environment.

However, pens and pencils, as well as hair oils and cosmetics, are not typically sources of lead poisoning.

Therefore, the correct answer includes smelting units and paints.