Nilfisk-ALTO BSG81 - page 12

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Replacing the HEPA filter (Depending on version)
If your vacuum cleaner is equipped with a HEPA filter, you need to replace this filter
once a year.
Replacing the activated carbon filter (Depending on version)
If your vacuum cleaner is equipped with a HEPA filter and an activated carbon filter,
you need to replace the HEPA filter yearly and the activated carbon filter twice a year.