Flash Arrestors are a tricky business. They are usually solid state devices that allow a fuel gas to flow through them, but do not allow a flashback to propagate back through. These usually contain some form of sintered filter, which acts as a diffuser and barrier. The sintering can be anything that is powdered, or porous. These materials have ranged from sand to fine bronze wool. Commercially made flash arrestors also contain a sintered metal filter. Unfortunately, they also contain thermal shutoffs. If the arrestor heats up, an internal mechanism shuts off flow. This poses a dangerous scenario for HHO enthusiasts, because HHO is produced on demand. This means if the individual doesn't pay attention, a dangerous level of pressure could build, increasing the risk of disaster.
The more a flash arrestor diffuses HHO, the less likely an ignition will pass through it. Many have reduced the particle size to as low as 1 micron in an effor to further diffuse the HHO, but this poses huge flow restriction issues. The more you restrict flow, the more pressure will build behind the arrestor; the more compressed the HHO, the more likely a flashback will pass through. So, a flash arrestor design is a bit of give and take. Currently, there is no mass manufactured flash arrestor for a mixed gas like HHO. Sure, manufacturers make them for Hydrogen or acetylene, but these are considered the "fuel gas".
They are a gas that would be harmless unless introduced to a necessary oxydizer, like Oxygen. Of course Oxygen exists in our atmosphere, which makes both Hydrogen and Acetylene dangerous when introduced to atmosphere. But contained in a bottle, they are considered "inert until oxydized".
The problem with HHO is that it contains both the fuel (Hydrogen) and its oxydizer (Oxygen) in the same hose or container. This poses a problem with flash arrestor manufacturers, because having both the fuel and oxidyzer in the same line can allow a flashback to travel explosively through most sintered materials.