Thread: Testing mask filter media

Thanks Bob.

For perspective on this topic, let me say that I keep up to date with Covid-19 developments via the New England Journal of Medicine, which seems to have updates each week.

The question that follows is whether masks are necessary or, rather, when are masks necessary?

They are obviously necessary if one has a cold or is coughing, and does not want to spread "anything". Being "breathed on" by someone infected but not symptomatic is a great concern many share. Who does one trust?

Masks are necessary if one was in a crowd, where many people are moving by and the 2m rule cannot be maintained. Especially in enclosed areas (although it seems that supermarkets are starting to limit numbers at a time).

Covid-19 is considered highly infectious, but mostly via touching an infected surface, and then transmitting it to one's face. Hence the recommendation for washing hands, etc. Of course, being coughed on would do it.

Having said all this, walking at a safe distance from others and visiting an uncrowded supermarket does not appear to be a risk factor that warrants wearing a mask to protect oneself. The question(s) is whether this is so, and whether the viral specialists are saying so because there are too few masks to go around?

Regards from Perth

Derek

Originally Posted by derekcohen

said all this, walking at a safe distance from others and visiting an uncrowded supermarket does not appear to be a risk factor that warrants wearing a mask to protect oneself.

The 1.5m spacing is highly arbitrary and depends very much on the specific situation. On a walking path or even at a bar with a mild cross breeze you could be right next to someone and not be affected. With a steady head wind walking for some time even 3m behind an infected person you could easily be walking continually in their viral plume. On a cycle path even 5m back could be enough to cop load.
I think this is what is contributing to the sporadic community spread of the virus as most stores simply don't have enough ventilation to address this problem.

The question(s) is whether this is so, and whether the viral specialists are saying so because there are too few masks to go around?

Here is something I wrote for a mens shed member who asked about masks.

I had a talk with X the other day and his daughter-in-law is one of the nurses or nurse manager types in the infectious ward at Z hospital and she apparently told him that the effectiveness of masks by the general public might even be more problematic than doing the social distancing thing.

At no stage do I suggest replacing the usual distancing with a mask. If we’re talking about persons such as kids, some teenagers, some grannies & gramps, and yobs (TGGY) I’d tend to agree that teaching people how to deal with masks. But its not rocket science for person with half a brain to learn how to handle, clean and store masks including when to wash your hands. I have years of experience with working in an ultra clean room where we used all that clean room gear , and while its the opposite (ie preventing human dirt/dust getting out and contaminating samples, rather than the other way) the principles are very similar. The educational requirements for teaching TTGYs how to use PPE properly can be demanding and confusing, which is probably why they don’t want to do it. However, the WHO website has plenty of clear and simple instructions - it's not that hard. I think (and rightfully so) the medical people are trying to stop people hoarding and wasting surgical type masks.When and how to use masks

Given that some people (including those who are ill) are going to wear masks anyway the very least the authorities could do is point them to the WHO website. This website also suggests that people who are well don’t need to wear masks but it does provide really important information for people who might think they are ill and should be wearing a mask.
In fact I think it is negligent for the Australian Health Dpet not to refer people who need to wear masks to the WHO website.

The way I look at it is like this. If you are well then there is indeed no need to wear a mask in lightly/low populated areas like large parks, or at home.
However, even with a 1.5m spacing, when walking/waiting near people, like a supermarket aisle or even on a busy foot path/cycle path you are constantly inside a serious plume of human breath vapour and potentially SARS-2. You can choose to have that plume continually impact your face AND breathe it straight in OR you can wear a mask and reduce the viral load you are exposed to.

Masks or face covering should not be worn for too long (as short as possible and no more than a couple of hours) before they are removed and cleaned, Cleaning is easy, hang them up and saturate then in meths and leave to dry.

I suppose you could work out how to do that with your cloth mask and transfer it straight into the washing machine to clean it, presumably in a hot wash cycle? I still think they are worthwhile as long as you are well informed about how to use them. Do you use a paper tissue inside the mask? Something that you can throw away each time you remove the mask?

Tissue does no filtering and just goes soggy. There’s info going round that suggests using a piece of Vacuum cleaner bag or filter, or even a piece of old T-shirt as a replaceable filter inside a double layer masks but my measurements suggest these do very little. I don't think the general public should use surgical masks and these should be left for the pros the NEED TEHM. If you are going to wear other masks it's much better to have several washable masks that you rotate around. have one spare, one that you are wearing and one being cleaned.

Thanks for that Bob. The bit about walking for leisure is very sobering. I've passed it onto my wife - not that I expect her to be impressed - quite oppositional defiant and sees me as paranoid! Don't say it!

Regards from Perth

Derek

Interesting seeing your 1.5 metre rule for social distancing. Here 2 metres are recommended and the government was up until recently saying masks were not necessary. Now they say to cover with a home made mask or use a scarf.

Public gatherings and events have been banned along with all non essential businesses being closed. Movies, clothing stores, gyms, sporting good, etc.

Takeout or prepaid delivery only for restaurants. Some businesses are taking orders and prepayment. When you arrive you stay in your car and open the trunk, call them and they come out and put the food in the trunk, close it and you drive off.

All preschool, elementary, high schools and colleges have been closed until next fall. All parks, playgrounds and campgrounds are also closed. The police are handing out fines for breaking the rules on gatherings.

I have a learned mistrust of government and generally when they say to not do something or to do something I do the opposite. I'm wearing masks and gloves if I must go out for essentials otherwise I am staying home.

Bob I had a thought. How much UV light does it take to sterilize a mask? Or does it at all?

Pete

Originally Posted by derekcohen

Thanks for that Bob. The bit about walking for leisure is very sobering. I've passed it onto my wife - not that I expect her to be impressed - quite oppositional defiant and sees me as paranoid! Don't say it!

Heres a very recent research article showing the spread of SARS-2 up to 4m away from a patient in a hospital
Early Release - Aerosol and Surface Distribution of Severe Acute Respiratory Syndrome Coronavirus 2 in Hospital Wards, Wuhan, China, 2020 - Volume 26, Number 7—July 2020 - Emerging Infectious Diseases journal - CDC

If it spreads 4m from a stationary patient in a hospital then a 5m plume emission by cyclists and walkers on a track starting to look even more likely.

Originally Posted by QC Inspector

Interesting seeing your 1.5 metre rule for social distancing. Here 2 metres are recommended and the government was up until recently saying masks were not necessary. Now they say to cover with a home made mask or use a scarf.
Public gatherings and events have been banned along with all non essential businesses being closed. Movies, clothing stores, gyms, sporting good, etc.
Takeout or prepaid delivery only for restaurants.
Some businesses are taking orders and prepayment. When you arrive you stay in your car and open the trunk, call them and they come out and put the food in the trunk, close it and you drive off.
All preschool, elementary, high schools and colleges have been closed until next fall.
All parks, playgrounds and campgrounds are also closed.
The police are handing out fines for breaking the rules on gatherings.

Pretty similar but with a couple of differences

People are strongly encouraged to stay home
BUT
Most retail stores can stay open but staff and customers must maintain the 1.5m rule.
Playgrounds closed but not many parks are closed and people from the same household can exercise together in the parks.
For takeout you can still go into some stores to collect food - customers to maintain 1.5m rule.
Child care and schools are still open mainly as babysitting services for parents that need them
We have both inter and intra(regional) state movement restrictions. eg In WA you cannot go to your holiday home in a nearby region. You have to demonstrate a need to travel between states and state regions. There are road blocks on main roads between regions.

Despite the comparatively loose restrictions above we still have a much lower and falling death and infection rates than Canada so something is working. In fact its working too well and some are baying for the lifting of restrictions.

Bob I had a thought. How much UV light does it take to sterilize a mask? Or does it at all?

My understanding is that UV from sunlight will only have about the same effect as free radicals in the air ie takes a few days. A UV lamp would do much better job for viruses on the surface but if the virus is buried in the cloth less so. A quick saturation in a water alcohol mix and the occasional hot water and detergent wash is all that is needed.

Originally Posted by QC Inspector

Bob I applaud you for where you are heading with your tests.

Would a large box or barrel with a hose to your cylinder of steel in which you blast in a measured amount of a known media, maybe a teaspoon of icing sugar, with compressed air work?

If you get it going would you mind testing two layers of Swiffer Sweeper cloth and if you have one, a piece cut from Miele vacuum cleaner bag?

The Miele vac cleaner bag is on the graph.

I asked SWMBO about Swiffers and she went into the midden (spare room) where there is a mountain of stuff left over to sort from he deceased mum's place and lo and behold came out with a pack!

Will test later today or tomorrow.

My guess as to why you have fewer people with the virus is you may have started sooner than we did. It wasn't until Trudeau's wife went to England and returned with some of the symptoms that the government got busy dealing with the pandemic. I think he got spooked by it and woke up. By then it had a greater foothold. Our neighbours to the south took even longer to get it together with some states waiting until a couple weeks ago to get started. They still have people going to church because God will protect them. If every one had acted at the end of January or early February rather than a month or more later this pandemic would't have been as far reaching and there would be fewer lives lost.

I'll stop now as I'm stepping over the dust pile and on to the political soapbox.

Pete

Has anyone else here looked into the 3D printable masks? They use replaceable filter media so I'm finding Bob's testing very interesting.

Dave is there a soft printing rubber so the mask conforms to the ever changing curves on our faces? Any links to examples of the ones that are being made?

My brain just doesn't want to get involved with the CAD and the programming side of CNC stuff. I do see the appeal and value of it though.

Pete

The ones I've seen use weatherstripping for a seal. Some designs have you use PLA and then heat form it to your face before adding the weatherstripping.

Here's a link to one of them. Google will get you lots more.

Yesterday arvo I tried to remeasure some of the filter media and found I was getting inconsistent results. It then dawned on me that I was running the pump ~2.5 times faster than a normal peak breathing rate so this morning I slowed the pump down by about 50% and got much more consistent results with the fabric type materials.

The P2 type filter media high efficiencies did not change but it made a difference to the coarser type fabrics.

Here is a one example.
The "Flannel" is the same material as used by the "Cloth mask" in the above graphs.
So performing the test at ~30 L/min at 2.5 microns gives an efficiency of ~ 85% compared to ~70% at 75L/min
The red and blue lines below are for a single(blue) and double (red) layer of the same cloth - the differences are not significant as the efficiency measurement tolerance is +/- 5% at these slower air speeds.

The grey and yellow lines are for single and double layers of "Swiffer" cleaning cloth.

Note how doubling the layers makes little significant difference - this sounds counter intuitive but is usually what happens with a fabric filter structures.

Screen Shot 2020-04-13 at 5.02.17 pm.png

I guess this clearly shows that even a cloth mask will provide some filtration.

Bob, what is the area of the test piece?

Originally Posted by dmorse

Bob, what is the area of the test piece?

The hole the air is being drawn through and the cloth is nominally covering is 40 mm in diameter.

I'd like to think its ana analog for a pair of nostrils and a partially opened mouth but it was just the first Forstner bit I grabbed that looked like a good size.
Also it matches the diameter of the hole in the large steel cylinder that holds the material over the hole.
Air flow through a real mask will be be over a wider area so filtration should be at a lower velocity and the filtration is likely to be better.