The term nanomaterials refers to materials that have at least one dimension (height, width or length) that is smaller than 100 nanometres (10−7 metre), which is about the size of a virus particle. This particular size dimension represents a major characteristic of manufactured nanomaterials (MNMs). The unique properties of MNMs may result in better paints, better drugs and faster electronics. However, for the same reason, MNMs may also present health hazards that differ from those of the substance in bulk form, and may require different test methods for hazard, exposure and risk assessment from their bulk material counterparts. There is currently a paucity of precise information about human exposure pathways for MNMs, their fate in the human body and their ability to induce unwanted biological effects such as generation of oxidative stress. Data from in vitro, animal and human MNM inhalation studies are available for only a few MNMs. So far, no long-term adverse health effects in humans have been observed. This could be due to the recent introduction of MNMs, the precautionary approach to avoid exposure and ethical concerns about conducting studies on humans. Health recommendations must, therefore, be based on extrapolation of the evidence from in vitro, animal or other studies from fields that involve exposure to nanoscale particles, such as air pollution, to the possible effects in humans. Workers in all countries will be at the front line of exposure to these materials, placing them at increased risk for potential adverse health effects. Therefore, WHO proposes these guidelines to policy makers and professionals in the field of occupational health and safety with recommendations on how best to protect workers from the potential risks of MNMs.