Aerosol filters.
This review discusses filter materials, filter mechanisms, filter testing and applications of filters in medicine.  While many applications are satisfactorily solved, there seems to be a research need for generating particle free air to relieve pains from people suffering from allergens and standardization of filter testing procedures for filters used in exhalation lines with humid air.
Hochrainer
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Boehringer Ingelheim KG, Ingelheim am Rhein, Germany.
D000336:Aerosols; D000393:Air Pollutants; D000485:Allergens; D005069:Evaluation Studies as Topic; D005374:Filtration / Q000295:instrumentation*; D006801:Humans; D008422:Materials Testing; D012134:Respiratory Protective Devices / Q000639:trends*