Definition of "ultrafine" []

• Very fine (adjective)

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• "Another 2005 study by researchers at the University of Texas found that when an ozone machine was turned on in the presence of a pine-oil cleaner, it produced "significant increases" in so-called ultrafine particles, a pollutant that is of concern to health experts because of the particles 'ability to penetrate deeply into the lungs."
• "They found that emissions of so-called ultrafine particles, which are less than 1/500th width of a human hair, were significantly elevated when compared to background pollution levels."
• "The study, published today in the journal Environmental Science and Technology, found that emissions of so-called ultrafine particles were 10 times higher than background levels about"