In the rush to make homemade masks, many people are wondering whether non-woven polypropylene bags are effective at filtering viruses. These bags are incredibly popular in places like Asia and Africa, where they are a common alternative to flimsy plastic shopping bags.
Why Non-Woven Polypropylene Bags Make Great Homemade Masks
These shopping bags are great candidates for masks because:
- They’re durable.
- Stores often give them away for free.
- They are easy to find.
In fact, in some African countries, markets hand them out for free to all shoppers. These bags in Uganda are real-life examples:
Thus, the bags are promising candidates, but how well do they filter out virus-sized particles? Can they really protect the wearer from viruses like the SARS-CoV-2 coronavirus?
What Are Reusable Shopping Bags Made From?
Reusable tote bags are made from a non-woven, polypropylene plastic (often referred to as ‘PP’). “Woven” materials are easy to understand. Cotton threads in our T-shirts are woven together.
The “non-woven” in the name means the plastic is bonded together. One way to do that is to melt the plastic so that it forms together. For example, imagine taking a handful of chocolate chips and melting them into a bar.
Non-woven polypropylene plastic is also the material used to make masks and HEPA filters. Since masks and HEPA filters can both capture the coronavirus, it would be reasonable to think that these bags might too. Thus, we put it to the test.
Testing Polypropylene Bags’ Ability to Capture Viruses
In the Smart Air lab, we tested 30 DIY materials including a polypropylene bag to see how well they these DIY mask materials could filter out virus-sized particles.
We measured the ability of the polypropylene bag and other materials to capture 0.3-micron particles and 1-micron particles. The SARS-CoV-2 coronavirus measures 0.06-0.14 microns by itself and 5-10 microns when in water droplets.
The polypropylene bag captured 22% of the smaller particles and 42% of the larger particles.
Doubling The Layers Boosts Effectiveness by One-Half
Next, we tested whether adding more layers would significantly increase effectiveness. The data showed that two layers of non-woven shopping bag material filtered 34% of small particles. Three layers more than doubled the particle capture.
How does that compare to masks? Effectiveness of 57% isn’t bad, but it’s still a far cry from the 97% the surgical mask we tested was able to capture.
Polypropylene Makes Decent Filters, But Very Breathable
Out of the 30 materials we tested, the polypropylene bag did not capture the most particles. However, it was one of the most breathable materials. A single layer bag was more breathable than a surgical mask and about as breathable as a 100% cotton T-shirt and bandana.
Three Layers Make Strong, Breathable Mask
Because it’s so breathable, polypropylene can form three layers and still be more breathable than a surgical mask. Thus, a triple-layer non-woven polypropylene mask makes a great candidate for homemade mask material.
Learn More: Discover which materials are best for making homemade masks in our ultimate DIY mask material review»
Paddy is the CEO of Smart Air, running operations from Beijing. He has a Masters in aeronautical engineering from Bristol University, UK having specialised in aerodynamics. An advocate for open data, free information and transparent business, he spends his spare time promoting honest business and social enterprise.