DIY air purifiers

There DIY air purifier movement has grown in recent years, largely due to the mark-up of air purifiers in an unregulated market.

Smart Air Filters, pioneers of the DIY purifier, began life as a social enterprise focusing on affordable clean air through education, research and DIY components for building your own – to date they have 20k+ customers of the DIY purifiers. Smart Air’s premise is that all you need for clean air is a powerful fan and a filter. By focusing on just these two elements, Smart Air went on to produce 3 incredibly efficient purifiers that are amongst the best in their class in terms of price, noise and clean air delivery. Smart Air continue to offer DIY products to ensure that price is never a barrier to clean air.

The DIY purifier trend reached the next level in the US when Dr Richard Corsi and Jim Rosenthal designed a purifier using the ubiquitous budget box fans and panel filters. At around US$70, the Corsi-Rosenthal (CR) box fans are low cost and extremely efficient, and in a head-to-head test, it outperformed a US$900 IQAir purifier. The CR box fan has attracted such a huge following in the US with many schools adopting the design as a low-cost method of providing clean air to students. For more information, visit Articles Archives - Tex-Air Filters ( or follow Jim Rosenthal on Twitter @JimRosenthal4.

So why hasn’t this taken off in the UK?

One reason – cost!

  • Filters: Those in the US with air conditioning are familiar with MERV rated panel/furnace filters. The best filter for the the CR box fan is the MERV 13 filter (captures up to 50% of PM0.3). The closest equivalent here, in terms of air filtration, is the F7 filter, which is a pocket filter rather than a panel filter. The highest rated EU panel filter is the G4 which is lower spec, equivalent to a MERV8. This leaves HEPA filters for UK/EU which are more costly.

  • Fans: The Smart Air fan requires a desk fan with a flat front – the cheapest one we found was around £30 and isn’t very powerful. The box fans are US made and even if you are lucky to find one in stock in the UK, it won’t be powerful enough for HEPA filters.

After several fan purchases, we think we may have cracked it 😊. Still not as cheap as the US version (or as pretty!) but we managed to create a CR box fan for under £80.

  • 18” ElectriQ Chrome floor fan – We purchased a grade A model which cost £30.73 including delivery (airflow is listed as 5000m3/hr or 3,600cfm but our anemometer read it higher than this on the slowest speed)

  • 4 x HEPA 12 filters – £44 including delivery on our site but do look around, see tips below for filter choice

  • Bottom box, 20” square box – re-use if you can, we purchased one locally for £4.32

  • Top box – Free as this was the box the fan came in

  • Tape – Free as we had this already (be warned that brown parcel tape will stink the first time you switch the purifier on, but the smell goes)

  • Total cost: £79.05

Here are the instructions for the 18” fan.

So does this work?

We don’t have the necessary equipment to measure pressure drops or CADR, but we do have a particle counter and an air quality monitor. The particle counter was placed on top of the fan where the ambient PM0.3 count was 1904. After 1.5 minutes, it went down to 970 and after 15 minutes, it was 90. Was this because the particle counter was right on top of the fan where the airflow blew away the particles? To be sure, we checked the air quality monitor which was 5m away on the opposite side of the room. PM2.5 went down to zero after 15 minutes!


There is a lot of information on Jim’s site (including tips and safety tests carried out) so do read up on this. The tips below are more for the UK/EU market in terms of the filters and fans available here:

  • MERV filters are furnace filters that are Medium Efficiency with courser fibres. HEPA filters are higher efficiency with tightly knit fibres to capture more particles. The higher the grade filtration, the lower the airflow due to more resistance with tighter fibres. If using HEPA filters, the fan must be more powerful than the fan used for furnace filters

  • The HEPA filters on our site are the Smart Air HEPA 12 filters used for their DIY purifiers. We have a limited supply and in line with Smart Air’s ethos of affordable clean air, we are subsidising these. Read Smart Air’s article on cheaper HEPAs

  • We needed to find a fan powerful enough to work with HEPA filters. After testing a few, we suggest a gym fan, also known as a high velocity floor fan. You can find them on Amazon, Wickes, Wilko, Homebase etc. However, the majority don’t provide noise levels and airflow which are important considerations. If it looks like the high velocity floor fan above and the power consumption is over 100 watts, it could be a high-powered fan but do your own due diligence and check with the supplier before you purchase. We only know the information on two brands – ElectriQ have it on their website and Senelux told us via their FB page that the approximate airflow is: 12 Inch Fans 3900m3/hr; 20 Inch Fans 7920m3/hr. We have ordered their 12” fan which we will test and report back here once it arrives

  • Be aware that a high velocity fan is likely to be noisier (57-68 dB). It is possible that a thicker cardboard box may be quieter

The last word…

If you do make a DIY purifier, please share on Twitter to both @airbonpurifier and @JimRosenthal4. Happy DIY-ing 😊