This is a guide for people who want to build their own air purifier with just a simple fan and a HEPA filter. This one has been experimentally proven to reduce harmful particulate pollution in real-world conditions. Tests have also shown that if you use a strong enough fan, you can build a purifier that is as effective as the big brand purifiers. (link to more about this here). The idea is to add a reliable HEPA filter in front of a flat-surfaced fan with decent wind speed. The air filter's cleaning capacity varies with the size of the fan and the type of the filter used; the tests were done using a Shinee fan (model: KYT25-15) with HEPA filter (HEPA class: H11) bought from Smart Air.

Step 1: Preparations

1 x HEPA filter

Don't scrimp on this as the quality of air you will get is highly dependent on it! Make sure it is bigger than the fan otherwise you will have air that leaks around the side of the filter.

1 x fan

The higher the air flow through the fan, the better.

1 x saw

You might not need this, depending on your fan model.This fan has a plastic knob that gets in the way of the HEPA, so i use the saw to cut the knob.

1 x needle nose pliers

Again, this depends on the type of fan you are using; you might not need it if your fan already has a flat front face. I use this to remove a piece on the grill to make the surface flat. If your fan has a flat front or back already, you're good to go!

1 x Velcro strap

Long enough to hold the filter and fan together.

Step 2: Removing the Cover

In order to create a flat surface on the front of the fan for the HEPA filter, you need to remove the cover from the fan. The cover also has an effect on the air flow. Watch out though, removing the cover will expose the fan blades, so if you've got any small fingers around; make sure to keep the fan out of their reach!

In the case of the Shinee KYT25-15, you need to first unscrew the green cap in the center of the fan. Then, you need pliers to remove the metal crescent tab from the center as seen in picture 2. Finally, you can remove the green fan grill!

Similarly, for any other fans, the main idea is to remove any covers/grills/blockages to maximise the efficiency of your DIY air purifier. Here are instructions for removing the front off a different fan. [link]

Step 3: Making the Fan Flat

For the Shinee KYT25-15, there is a protruding white bit of plastic in the middle of the fan after you've removed the grill. To remove the plastic part, you'll want to use a saw to saw it off. Once done, the flat surface created is perfect for attaching the HEPA filter on top of the fan (see the second picture).

For any other fans, this step may not be necessary. It is fine as long as the surface is flat enough to place the HEPA filter.

Step 4: Finishing Touches

Last step! Place the HEPA filter on the fan, and you're done.

HEPA filters are design to have air flow in one specific direction. Ensure you have the correct side of the HEPA filter attached to your fan. You can check it by feeling the filter itself (as I've done in the second picture). You'll feel one 'furry' side and one 'hard' side. The air should flow through the furry side and out of the hard side. But, not all HEPAs are the same. This method of finding out the correct side may fail at times. Hence, its best if you check with your suppliers in order to assemble correctly!

For tightening the HEPA filter to the fan, a Velcro strap is great since it allows you to easily remove the filter when it's time to replace. However, if a Velcro isn't convenient, you could use any other method (like duct tape). Here's a picture of the first DIY fan Smart Air ever made - it used a tape measure to strap the HEPA filter on to the fan!

Step 5: Add-ons

HEPA filters capture particulate air pollution; that's the really tiny particles of which PM2.5 is a part of (to know more about Particulate matter). One thing they don't capture is gases. If you think your house has smells or a formaldehyde problem (nerd note: VOC gases), you could opt for a carbon filter instead of a HEPA filter. These filters work by adsorbing the VOC gases; remove them entirely from the air.

Bear in mind you might not need a carbon filter though. If you need to filter both gases and particulates I would recommend using both filters. However, the efficiency of your DIY air filter may suffer, as found in these tests. Thus, you can either get 2 fans with different filters, or one that has an extra high wind flow.

Step 6: FAQ

longevity of filters

This depends on the quality of filter used, how polluted is the air you were trying to clean and how often you use your purifier every day. Smart Air did experiments in Beijing and found that the HEPA filters used in this instructables post last around 1,000 hours before their effectiveness fell meaningfully. Of course, if you live in a place that has cleaner air than Beijing (which you probably do!), the longevity of filter will be longer.

Where should i place the filter?

Many people believe I should have placed the HEPA filters behind the fan instead of the front as this will improve it's effectiveness. However, we have done some tests and found out there isn't a significant difference. In fact, by placing the filter at the back of our Cannon purifier, it actually performed slightly worse! (read up on this Quora answer by Thomas Talhelm to find out more)