Wikipedia:Reference desk/Archives/Science/2021 July 30

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July 30

What's the maximal level of absolute humidity possible practically?

What's the maximal level of absolute humidity possible practically & naturally on earth (troposphere)? --ThePupil (talk) 03:25, 30 July 2021 (UTC)

This is dependent upon a few factors, mainly temperature (which is not at all uniform in the troposphere). At 25 °C, water has a vapor pressure of 3.169 kPa. Above this pressure, water vapor will condense to the liquid state, so this pressure can be taken as a maximal pressure. At this temperature and pressure, water can be approximated as an ideal gas, which means we can use the ideal gas law (pV=nRT) to approximate the concentration of water the air (n/V=p/RT), which gives us a concentration of 0.00128 moles of water per liter of atmosphere. Using the molar mass of water, that gives us ~0.0231 grams of water per liter of atmosphere, or 23.1 mg of water per liter of atmosphere. Again, that's at roughly room temperature. Since the vapor pressure of water is temperature dependent, and temperature is also a variable in the ideal gas law, we can expect a fair amount of temperature dependent variation in this number. For example, at 55 °C, among the hottest ever recorded surface temperatures, this concentration would be 104 mg of water per liter of atmosphere, while at at the coldest recorded temperature (roughly -90 °C), the concentration would be essentially 0 mg per liter of atmosphere, as the vapor pressure of water is practically zero once you go below the freezing point. Again, these are approximations, but should be fairly good. Note that clouds will contain a much higher concentration of water than that given here, but that is because clouds contain most of their water in the liquid state. Humidity is explicitly the measure of water vapor content in the air, and not water liquid content. --OuroborosCobra (talk) 04:50, 30 July 2021 (UTC)

Dew points of a little over 30°C have been recorded in places like India, the Amazon and around the Persian Gulf. This gives a vapour pressure of around 5 kPa, so that would be around 30 mg per litre, using the answer above. In higher temperatures, the dew point could be higher, but in reality this doesn't happen as much higher temperatures don't occur in places where water is available for evaporation. Evaporation takes enough energy to prevent such temperatures from occurring. PiusImpavidus (talk) 09:06, 30 July 2021 (UTC)

35°C if you narrow it down to single place, single time. Almost 40mg/liter or 0.04 in the kg/m³ units below.Sagittarian Milky Way (talk) 19:41, 30 July 2021 (UTC)

What's the density of steam at 100°C and standard atmospheric pressure? Roger (Dodger67) (talk) 09:33, 30 July 2021 (UTC)

According to Water (data_page)#Water/steam equilibrium properties, about 0.6 Kg/m³. Mike Turnbull (talk) 11:27, 30 July 2021 (UTC)

Well, that doesn't occur naturally as a meteorological phenomenon in the troposphere, but one can argue it occurs naturally in fumaroles. PiusImpavidus (talk) 15:52, 30 July 2021 (UTC)

PiusImpavidus, it is however a regular occurrence in billions of kettles and cooking pots every day. Roger (Dodger67) (talk) 21:39, 30 July 2021 (UTC)

What is the efficacy of a previous COVID infection against another infection?

The Pfizer vaccine, for example, is over 80% effective after six months but less than 40% against the Delta variant, according to Israeli studies. Are there similar studies for COVID patients who have recovered? 161.185.160.74 (talk) 21:07, 30 July 2021 (UTC)

Here are some primary sources (some more recent than others, be sure to contextualize each with its date--there's a lot more recent work out there, but I tried to curate a reasonable sized shortlist here to address several different aspects of the empirical question of post-infection immunity. This is, needless to say, an area of ongoing inquiry with a lot of open questions): [1],[2],[3], [4],[5],[6],[7],[8],[9], [10],[11].

And here are some secondary sources, some consisting of news/editorial commentary from peer review journals, others from a variety of news media summarizing, as best each can, the current state of research:[12],[13],[14],[15],[16],[17],[18],[19],[20].

Our WP:SYNTH policies prevent me from extrapolating a generalized figure from the primary sources: I recommend looking at them each in turn, again with the caveat that they come from all over the last nine months or so, and this being an area of swiftly advancing research. However, I can summarize some general points from the primary and secondary sources collectively: in short, there is cautious optimism for a substantial (but far from guaranteed) level of immunity for those who have previously been infected, but because long term data is (by definition) constrained by the length of the pandemic to date, the duration of this immunity is a big unknown. Furthermore, recent viral variants significantly complicate the question, and broadly speaking, researchers and public health officials tend to recommend vaccination for even those who have previously been infected, all other factors being even. More nuance in interpretation you will have to derive from your own review of the materials. SnowRise ^{let's rap} 01:49, 31 July 2021 (UTC)

I look at r/coronavirus sometimes. A lot of good info is there, along with a fair amount of not-so-good, but at least it's all in one place. Before Delta they were telling people who had not been infected to get two shots (Pfizer/Moderna), and people who had recovered from infection to get one shot. More recently I've heard people over 60 who have already had two shots are being advised to get a third shot. Some people with long term symptoms after recovering from infection (long covid) have experienced some relief from the symptoms after receiving vaccine. Besides vaccinations, it is still important to mask up, stay away from crowds, etc. The US govt made a big error in betting everything on vaccines: it desperately wants the pandemic to be over, but the virus is not so easy to persuade. 2601:648:8202:350:0:0:0:2B99 (talk) 04:18, 31 July 2021 (UTC)

The issue here is defining "efficacy". Efficacy has many meanings in medicine - not just vaccines. Some measures of efficacy that have been and/or could be used to describe vaccines include: risk of infection, risk of severe infection, risk of hospitalization, risk of death, risk of sequelae after infection resolves, (decrease in) viral load, "transmissibility", and many more. This is why it's never sufficient to say "well the Pfizer vaccine has an efficacy of 95% and the J&J has an efficacy of 85%" - because I just gave you the number for all infections for the Pfizer vaccine, and for severe infections for the J&J vaccine. For the purposes of this answer, I'm going to assume you're interested in any efficacy information, and I'll make it clear what the efficacies are describing.

• doi:10.1016/j.cell.2021.06.02: Convalescent serum (i.e. antibodies in blood from people previously infected) of people who had "regular" COVID, alpha variant, beta variant, and gamma variant, was tested to see its "strength" at neutralizing the delta variant, and this was compared to the serum's strength at neutralizing a very early sample (from early 2020, i.e. not mutated). Neutralization was decreased by anywhere from about 2-fold to about 6-fold depending on the exact delta variant tested. This basically means that antibodies produced from natural infection are somewhere between 16% and 50% effective at neutralizing the delta variant in vitro. This does not necessarily translate to infections in humans though. Of note, beta-variant antibodies were over 10-fold less effective at neutralizing the delta variant as they were at neutralizing the beta-variant. The same is true for the gamma variant. As such, it is in my opinion not too far a leap to say that people who previously were infected with the beta or gamma variants are likely not very protected against re-infection with the delta variant.
• doi:10.1038/s41586-021-03777-9: Similar results - convalescent serum (i.e. antibodies in blood from people previously infected) was 4- to 6-fold less neutralizing against the delta variant. By 12 months after infection, less than half of people were producing antibody levels that could neutralize the delta variant (compared with almost 90% for the alpha variant).
• doi:10.1093/jtm/taab104: 4.6x less neutralizing against delta than against "regular" COVID.

I stopped looking once I found 3 decent-quality articles that all basically said the same thing - in the average person who was infected with COVID-19, it requires 4-5x more antibodies to neutralize the delta variant as it did/would to neutralize "regular" COVID - or even some other variants. Thus their risk of infection is likely greater - but how much greater cannot be said with any accuracy as titers and "test tube tests" don't tell the full story. Obviously you will also have people who formed stronger immune responses to COVID to begin with, and as such are producing more antibodies than someone who had a very mild and quick infection. Unfortunately, for us to get actual "real life" data so we could say exactly how much more likely a re-infection with delta is than a re-infection with another variant (or original COVID), we need massive amounts of genetic sequencing. Not just for new cases, but we need to go back and sequence old cases - from fall 2020 onwards - to know which variant those people were infected with prior. Without knowing the original variant someone was infected with, we cannot begin to say how effective prior infection is at preventing delta-variant infection.

Not to mention that "prior infection" could mean infection in March 2020, which would have been the original variant, or it could mean infection with the delta variant three months ago. Re-infection of the delta variant after prior infection with the delta variant is unlikely - as the body will have produced antibodies specific for the delta variant. However, other variants will all have different levels of protection against re-infection with delta instead - so it's really complicated.

TLDR: antibodies in convalescent serum are about 4-fold less neutralizing against delta than against other variants - but we won't know for a few months likely how this actually impacts reinfection risk. -bɜ:ʳkənhɪmez (User/say hi!) 19:38, 31 July 2021 (UTC)