Infographic illustration depicting the relative size of particles. A strand of human hair (far right) measures 50-80 micrometres in width which is similar to particles, such as salt and sand, that are visible to the naked eye. White blood cells and pollen grains are smaller and not visible to the naked eye. The smallest particle in this image is a Zika virus, which measures just 0.045 micrometres (45 nanometres) in size. Dust, dirt, soot, and smoke particles found in air pollution measure around 2.5 micrometres and are a leading cause of health problems. Wildfire pollution particles are smaller (04 - 0.7 micrometres) and also cause health problems.