Azure Jane Lunatic (Azz) 🌺 (![[personal profile]](https://www.dreamwidth.org/img/silk/identity/user.png)
azurelunatic) wrote2020-05-27 04:27 pm
azurelunatic) wrote2020-05-27 04:27 pm
Entry tags:
Covid-19 reference, so I can close some tabs
https://www.cebm.net/covid-19/question-should-smartphone-apps-be-used-as-oximeters-answer-no/
Key quotes:
THE SCIENTIFIC BASIS OF OXYGEN SATURATION MEASUREMENT
Oxygen saturation is the fraction of oxygenated haemoglobin relative to the total haemoglobin (oxygenated + deoxygenated) in the blood.
SpO2 = [HbO2] /([Hb] + [HbO2])
where HbO2 is oxygenated haemoglobin and Hb is deoxygenated haemoglobin.
The measurement of SpO2 with a pulse oximeter relies on the fact that the two forms of the haemoglobin molecule, Hb and HbO2, have light absorption properties which vary with wavelength in the visible and infra-red parts of the spectrum. It therefore requires the measurement of light transmission or reflection from a body segment such as a finger at two different wavelengths (usually in the red and infra-red). SpO2 cannot be measured with a smartphone flash light and camera, since the latter cannot measure light reflection at two discrete wavelengths.
...
if we assume that they are claiming an error of ±4%, then a random number generator with a mean value of 95% and errors randomly distributed between -4% and +4% would give values between 91% and 99%
...
The Samsung Galaxy series of phones had a red light emitting diode (LED) built into the phone in addition to the flash light and camera. There were no details released by the company of how its app used the LED to estimate oxygen saturation, but it appears from publicity material on YouTube that it worked via a single-wavelength measurement (albeit with a monochromatic light source, the LED) and therefore that oxygen saturation could not be accurately derived from it.
The methodology used by Tayfur and Afacan in their study is sounder than in the Tomlinson study because their reference device is an arterial blood gas (ABG) analyser. The Bland-Altman plot (Figure 4) in their paper shows that most of the oxygen saturation measurements from their Emergency Department patients were between 95% and 100%. For the few patients whose oxygen saturation measurements were between 85% and 93%, the difference between the smartphone estimate and the ABG device varied between -5.5% and +2.5%. In other words, the readings become less accurate as the patient becomes more hypoxic.
Samsung withdrew its claim of being able to measure oxygen saturation in May 2019 with its built-in red-light source in Galaxy phones and instead now makes the false claim that “you can measure your oxygen saturation level by measuring your stress” ....
https://www.doh.wa.gov/Emergencies/Coronavirus
Dashboard: https://www.doh.wa.gov/Emergencies/NovelCoronavirusOutbreak2020COVID19/DataDashboard
https://pubs.acs.org/doi/10.1021/acsnano.0c03252?goto=supporting-info
Guidance
We highlight a few observations from our studies for cloth mask design:
Fabric with tight weaves and low porosity, such as those found in cotton sheets with high thread count, are preferable. For instance, a 600 TPI cotton performed better than an 80 TPI cotton. Fabrics that are porous should be avoided.
Materials such as natural silk, a chiffon weave (we tested a 90% polyester–10% Spandex fabric), and flannel (we tested a 65% cotton–35% polyester blend) can likely provide good electrostatic filtering of particles. We found that four layers of silk (as maybe the case for a wrapped scarf) provided good protection across the 10 nm to 6 μm range of particulates.
Combining layers to form hybrid masks, leveraging mechanical and electrostatic filtering may be an effective approach. This could include high thread count cotton combined with two layers of natural silk or chiffon, for instance. A quilt consisting of two layers of cotton sandwiching a cotton−polyester batting also worked well. In all of these cases, the filtration efficiency was >80% for <300 nm and >90% for >300 nm sized particles.
The filtration properties noted in (i) through (iii) pertain to the intrinsic properties of the mask material and do not take into account the effect of air leaks that arise due to improper “fit” of a mask on the user’s face. It is critically important that cloth mask designs also take into account the quality of this “fit” to minimize leakage of air between the mask and the contours of the face, while still allowing the exhaled air to be vented effectively. Such leakage can significantly reduce mask effectiveness and are a reason why properly worn N95 masks and masks with elastomeric fittings work so well.
Conclusions
In conclusion, we have measured the filtration efficiencies of various commonly available fabrics for use as cloth masks in filtering particles in the significant (for aerosol-based virus transmission) size range of ∼10 nm to ∼6 μm and have presented filtration efficiency data as a function of aerosol particle size. We find that cotton, natural silk, and chiffon can provide good protection, typically above 50% in the entire 10 nm to 6.0 μm range, provided they have a tight weave. Higher threads per inch cotton with tighter weaves resulted in better filtration efficiencies. For instance, a 600 TPI cotton sheet can provide average filtration efficiencies of 79 ± 23% (in the 10 nm to 300 nm range) and 98.4 ± 0.2% (in the 300 nm to 6 μm range). A cotton quilt with batting provides 96 ± 2% (10 nm to 300 nm) and 96.1 ± 0.3% (300 nm to 6 μm). Likely the highly tangled fibrous nature of the batting aids in the superior performance at small particle sizes. Materials such as silk and chiffon are particularly effective (considering their sheerness) at excluding particles in the nanoscale regime (<∼100 nm), likely due to electrostatic effects that result in charge transfer with nanoscale aerosol particles. A four-layer silk (used, for instance, as a scarf) was surprisingly effective with an average efficiency of >85% across the 10 nm −6 μm particle size range. As a result, we found that hybrid combinations of cloths such as high threads-per-inch cotton along with silk, chiffon, or flannel can provide broad filtration coverage across both the nanoscale (<300 nm) and micron scale (300 nm to 6 μm) range, likely due to the combined effects of electrostatic and physical filtering. Finally, it is important to note that openings and gaps (such as those between the mask edge and the facial contours) can degrade the performance. Our findings indicate that leakages around the mask area can degrade efficiencies by ∼50% or more, pointing out the importance of “fit”. Opportunities for future studies include cloth mask design for better “fit” and the role of factors such as humidity (arising from exhalation) and the role of repeated use and washing of cloth masks. In summary, we find that the use of cloth masks can potentially provide significant protection against the transmission of particles in the aerosol size range.
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