Respirable suspended particulate matter, of less than 10 micrometers (µm) in size, can be present in the drift from cooling towers. Larger particles above 10 µm in size are generally filtered out in the nose and throat via cilia and mucus but particulate matter smaller than 10 µm, referred to as PM10, can settle in the bronchi and lungs and cause health problems. Similarly, particles smaller than 2.5 µm, (PM2.5), tend to penetrate into the gas exchange regions of the lung, and very small particles (less than 100 nanometers) may pass through the lungs to affect other organs. Though the total particulate emissions from wet cooling towers with fresh water make-up is much less, they contain more PM10 and PM2.5 than the total emissions from wet cooling towers with sea water make-up. This is due to lesser salt content in fresh water drift (below 2,000 ppm) compared to the salt content of sea water drift (60,000 ppm).
