Research done in 2014 found that yellow dust consists of fine dust and ultrafine dust particles. Fine dust consists of fine particular matter (PM). Particles smaller than 10 µm in diameter are classified as fine PM (PM10), while particles smaller than 2.5 µm in diameter are classified as ultrafine PM (PM2.5). Both fine and ultrafine dust particles impose dangers to health. Fine dust particles are small enough to penetrate deep into the lung alveoli. Ultrafine dust particles are so small that they also penetrate into the blood or lymphatic system through the lungs. Once in the bloodstream, ultrafine particles can even reach the brain or fetal organs.