Fume extractors are not self-cleaning. A thin layer of dust on the internal surfaces of the hose or pre-filter section of the extractor is normal. However, you should be on the lookout for excessive accumulation of dust or debris in the extractor and hoses as this could cause air blockages. Also, be on the lookout for any dust downstream of the HEPA filter. This should not occur unless your HEPA filter has been damaged.
To clean out excessive dust and debris from the cabinet or hoses, wear appropriate personal protective equipment, such as latex gloves, a dust mask, and goggles. Use a vacuum cleaner equipped with a HEPA filter to suction out the contaminants. Do not use the vacuum cleaner on the filters. The direct application of a point source suction on the surface of the filter may damage the filter.