Miasma (μίασμα) is one of the things I see getting confused and discussed regularly in this community (on Tumblr and otherwise)
The two main streams of thought seem to be either μίασμα is something you can acquire from everyday life and thus needs to be cleansed before coming into contact with the Gods or μίασμα is unnatural and can only be acquired through severe acts like murder or desecration of a temple and the thing that does need to be cleansed is Lyma (λῦμα) which is essentially physical dirt.
These two ideas on what μίασμα is are very clearly contradictory so I wanted to make a post to clear the misconception up with sources to hopefully encourage further research on the topic. (I would encourage you to do so, and to correct me if I've misinterpreted something.)
The first source I looked at was the “Purity regulations at Cyrene” a fourth century BC inscription
which lays out some very clear rules on what does and doesn’t constitute μίασμα.
An example of these rules would be:
“[ἁ λ]εχὼι ὄροφομ μιανεῖ· τὸμ μ[ὲν ὑπώροφον μιανεῖ, τὸν]
[δ᾽ ἐ]ξόφορον οὐ μιανεῖ, αἴ κα μὴ ὑπένθηι· ὁ δ᾽ ἄ[νθρ]-
[ω]πος, ὅ κα ἔνδοι ἦι, α⟨ὐ⟩τὸς μὲν μιαρὸς τέντα[ι ἁμ]-
[έρα]ς τρῖς, ἄλλον δὲ οὐ μιανεῖ οὐδὲ ὅπυι κα ἔνθ[ηι ο]-
“[The woman] in childbirth will pollute the roof (i.e. the house): [she will pollute] one [who is under the roof, but] not the one outside the roof, unless he goes in. Any human being who is inside (the house) will himself be polluted for three days, but will not pollute another (person), wherever (20) that human being may go.”
In this extract the verb μιανεῖ (to stain) is used which the word μίασμα derives from (I did want to get a source on this since i’ve only started learning ancient greek relatively recently but I couldn’t find anything concrete to link but even without a source I think this can be assumed given the similarity of the two words.) So from this we can tell that μίασμα is something that can be gained from more everyday or morally neutral activities for example childbirth can cause μίασμα but also that μίασμα is also contagious.
I then looked at a few similar inscriptions on this website (Collection of greek ritual norms - http://cgrn.ulg.ac.be) which I highly recommend. It includes both English and the Greek and has a wide array of sources to do with ancient Greek ritual.
Most of these inscriptions had very similar rules set out.
Another fourth century BC inscription “Regulation concerning purity and supplication from Metropolis” forbids people from entering the sanctuary for a number of days after either attending a funeral or having sex.
“One is [pure] from a [funeral after twelve] days, from a woman, if it is (5) [one's own], after two days, from a prostitute after three. One should not drag [a suppliant] away while he stands [by an altar], (10) nor [do] him any injustice. To the one who commits an injustice, may Meter Gallesia not be kindly to him.”
A 1st century BC inscription concerning the sanctuary of Artemis Kithone at Miletos which dictates two full days until you may enter the sanctuary after a funeral, childbirth or the birth of puppies and for other unnamed sources of contamination just washing is deemed to be ok.
“[Enter pure] into the temple of Artemis Kithone: from a funeral and from a woman giving birth and from a bitch that has given birth, on the third day having washed; from the rest, on the same day having washed.”
A second century BC inscription from a sanctuary to Syrian deities in Delos. These deities where likely Hellenized during this period. It includes the usual death and childbirth as things that can cause μίασμα but it also lists fish, swine and menstruation.
“With good fortune. Enter (into the sanctuary), being pure from fish, on the third day; from swine, having washed (on the same day); from a woman, on the third day; from a woman who has given birth, on the seventh day; from miscarriage (or: abortion), on the fortieth day; from female matters (i.e. menstruation), on the ninth day.”
I won’t go on to list all inscriptions I read but the one thing I found across the board is that there is absolutely not one set of rules that applied to every sanctuary in the ancient Greek world; each one would have its own different set of regulations.
But death and childbirth are both very common causes.
Now it’s all well and good knowing that μίασμα wasn't reserved for murder or extreme acts and that what constituted as μίασμα varied by sanctuary but where does that leave us as people in the modern day who don’t have access to sanctuaries? How much did μίασμα affect daily praxis?
The short answer is as far as I can see, we don’t really know. I definitely don’t think we as Hellenic polytheists need to concern ourselves with abstaining from prayer while menstruating or after having eaten fish since they seem to have been more niche rules for specific sanctuaries.
But I also don’t think we as worshipers should abstain from praying after someone within our house has gone through childbirth or the period after someone has died, unless that feels right to your practice.
But whatever you decide feels right it is very important that you wash before prayer.
“Never omit to wash your hands before you pour to Zeus and to the other Gods the morning offering of sparkling wine; they will not hear your prayers but spit them back.”
Hesiod 'Works and Days', lines 722-725
Ultimately μίασμα should not be something to be overly worried about (see Theophrastus’s the superstitious man) just wash your hands before you pray and you’ll be fine.