HEPA vacuums are expensive – not just to buy, but to operate. Unlike common wet/dry vacs, which drop debris directly into the canister, HEPA models use a series of bags and filters to capture debris. The first step in the series is a bag, which in addition to collecting debris serves as a prefilter that extends the life of the expensive HEPA filter behind it. In industrial models, there are additional prefilters between the collection bag and the main filter. The prefilter fits around the HEPA filter and is supposed to capture the fine dust that gets through the collection bag.
Bags. Most vacuums take paper bags, some take fleece bags, and a few take both. Paper tends to clog more quickly than fleece. Here's why: When a vacuum comes on, the bag – whether paper or fleece – inflates; when the vacuum goes off, a paper bag will maintain its shape, but a fleece one, being more flexible, will collapse a bit. The next time the vacuum comes on, the fleece bag will snap back to its fully inflated shape, causing the caked-on dust to crack and fall in with the rest of the debris.
Fleece bags also cost more, but they may be more economical to use because they can be filled more completely without losing suction. Plus they're tougher than paper, so they're less likely to break when you take them out of the machine.
When you're choosing a vacuum, you should consider the price of bags because over time you'll go through a lot of them. Filters, by contrast, last a very long time, so replacing them is less of a concern. It's best if you can buy consumables locally, because even if you normally buy online there may be times you can't wait for delivery.
When debris enters a HEPA vacuum, it's captured in a collection bag – usually paper (above) but sometimes fleece (below). Fleece bags cost more but are tougher and can be filled more completely without a loss of suction.
Filters. Some of the lead particles that come into the vacuum will be small enough to pass through the collection bag; the HEPA filter is intended to capture those particles and prevent them from escaping in the exhaust air. Since HEPA filters are expensive, it's good that the bags catch most of the debris and all but the smallest particles. The Bosch, Metabo, and Hilti vacs have an additional means of maintaining suction – built-in "shaking" devices that periodically knock dust out of the filters. This feature is typically found on vacuums designed to collect dust from tools used to drill or cut concrete and masonry.
If the filter fits poorly, dust will get by it and escape with the exhaust air. This often happens with budget-price vacuums – which is why the EPA and these machines' makers say not to use them for lead work, even when retrofitted with HEPA filters. The filter should be designed to fit that particular machine and should seat tightly against a gasket. Abatement contractors – who are subject to standards stricter than the RRP – use vacuums containing oversize filters that bolt or clamp tightly into the machine. These filters are individually tested and certified for compliance with a recognized standard such as MIL-STD-282 or EN 1822. The vacuum may contain a sensor that prevents it from operating with an incorrectly installed filter.