<b>What are Millton PM2.5 filters made of?</b><br> The Millton PM2.5 filter consists of five layers of activated carbon filter and different kinds of fabric:<br> *Layers 1 and 5: Spun-bound non woven material that filters out larger particles<br> *Layer 3 (middle): activated carbon layer. Traps particulate matter via an adhesion process.<br> *Layers 2 and 4: high-efficiency melt-blown filtering material that can filter small substances.<br><br> <b>How is a PM2.5 filter used ?</br><br> It is really simple to use this filter.<br> <b>What side of PM2.5 Filter faces out?</br><br> It does not matter which side of the PM2.5 faces out. The filter is constructed in a symmetrical manner such that regardless of which side faces out, you are always having the same two layers of fabric on either side of the middle carbon-activated layer (see pictures above). However, keep in mind that once used, it is recommended that you always have the same side facing out if you are going to use it multiple times.