You can find the complete WHO guidelines for indoor air quality in this link. The guidelines include exposure levels to various substances that can cause health concerns. The substances considered were: - Benzene - Naphthalene - Formaldehyde - Carbon monoxide - Nitrogen dioxide - Radon - Tetrachloroethylene - Trichloroethylene - Polycyclic aromatic hydrocarbons You can also find a summary of the guidelines here.