When I first started out with Baring the Aegis, one of the first posts I did was on miasma and katharmos--pollution and purification, respectively. The post can be found here. Nearly a year later, I stand behind what I wrote in that post, but it's time for a revisit. Today, I'm talking about katharmos and miasma, the importance they had in ancient Hellenic religion, and the importance they have in its modern equivalent. From the previously linked post:

"Within Hellenic practice, miasma describes the lingering aura of uncleanliness in regards to a person or space through which contact is made with the Gods. Miasma occurs whenever the space or person comes into contact with death, sickness, birth, sex, excessive negative emotions and bodily fluids. It also comes from a lack of contact with the Hellenic Gods. Not the actual acts of dying, sex and birth cause miasma but the opening up of the way to the Underworld (with births and deaths) as well as contact with sweat, blood, semen, menstrual blood and urine pollutes us. Miasma is an incredibly complicated and involved practice and it's often misunderstood. The most important things to remember about miasma is that it holds no judgment from the Gods, and that everyone attracts miasma. It's a mortal, human, thing."
"The practice of purification is called katharmos (Καθαρμός). The process of katharmos is elaborate because the process not only involves the physical but also the emotional, mental and spiritual.
The practice of katharmos historically starts with a bath (or shower, in modern times). Step two is the preparation and use of khernips (Χἐρνιψ). Beyond the practical, there is a large mental component to katharmos. It means leaving behind negativity, worry, pain and trouble before getting in contact with the Gods."
There has been quite a bit of talk about miasma lately. Dver wrote a rousing piece on it a couple of weeks back which I read with great interest, and mostly agree with. Ruadhán continued the trend with a fabulous rant, which I also largely agree with, so please, if this is a subject of interest to you, read these people's post in addition to my own. It might shed more light on the subject matter.

The greatest barrier in understanding miasma and katharmos, to me, is our modern frame of mind. On the one hand, we know too much about personal hygiene, about the human body and about science as a whole, on the other hand religion in general has become something separate from life in general. As a result, we color ancient Hellas with our 'hygiene brush'. Secondly, not everyone has faith, our society does no longer revolve around it, and as a result, we--as modern religious people--struggle for a mindset of simple, all-encompassing, unquestionable worship. There might be a few remnants of 'Original Sin Thinking' lodged in there as well.

We all incur miasma, every single day of our lives. It has nothing to do with sin, shame or guilt. Miasma is a consequence of living. We breath, make decisions, come in contact with others, and along the way, we become too human--for lack of a better term--to petition the Gods. The divide between the purity and cleanliness of the Theoi and our human mortality and imperfection, keeps us away from Them.

Miasma is not about being physically dirty, although that is a part of it, and katharmos is not about becoming physically clean, although that is a part of it. Like Ruadhán rightly points out, men would sometimes come to public rituals fresh off the fields, dirty, sometimes cut up and scraped, in rumpled daily wear. With a washing of the hands, and the sprinkling of the body, they would be considered clean from the daily miasma, although they were almost as physically unclean as they were before their cleansing. I would argue, though, that was more exception than rule, as state festivals were huge affairs that people prepared for for days--sometimes weeks--and wearing your best outfit was usually part of that preparation. For many of the tragic festivals, people even dressed up as something else. From Harrison's pre-Olympic themed 'Themis: a Study of the Social Origins of Greek Religion' comes the following reflection on a procession exhibited by Ptolemy Philadelphos in honour of Dionysos:

"The procession was headed by Silenoi clad, some in purple, some in scarlet, to keep off the multitude; next followed twenty Satyrs bearing lamps; next figures of Nike with golden wings; then Satyrs again, forty of them, ivy-crowned, their bodies painted some purple, some vermilion."
In researching this post, I pulled out some of my most trusted books on ancient Hellenic religion, from Parker to Harrison, to Mikalson, and of the ten books I gathered, none made note of miasma or katharmos in the index. To get that information, you need to buy a dedicated--and expensive--book on the subject. In reading these books, one finds mention of 'sprinkling with water' before a state ritual. I have discovered, though, that 'pollution' is often in the index, and miasma is meant with it, so keep an eye out for that. The fact that you have to search the indexes of scholarly works for any mention of miasma and katharmos completely blows past the importance that was placed upon these in the past. I think that is a problem, honestly.
Katharmos is devotional. It not only helps you get in a ritualistic mood, it prepares the room and your body for it. Even if you do not understand the use, it's a vital part of Hellenistic worship. I would advice any Hellenist to invest time in researching miasma and katharmos, however, as a large part of its effectiveness lies in understanding the practice. I will give you an example of the influence katharmos has on me:
I live a very busy, hectic, life, and most of my labour is mental. I get to work in the garden on occasion, but between college, a large number of projects, and the blog, I do most of my work with my brain and fingers, behind a computer screen. During busy times--which is nearly always--I work eighty hours a week on everything I have to do or feel I need to do, and all of it is behind a computer. That means that I'm behind this thing at least 11 hours a day; usually longer on three or four days a week, because I have appointments the other days of the week. By the time I get off of the computer at night, my back hurts, my head is swimming, and I'm exhausted. I perform my night time rituals before heading to bed. As soon as I start preparing for them, my mind clears. When I wash my hands, the tension drains out of my body, when I wash my face, the frantic pace of my brain slows down. I wash my face three to six times, depending on how stressed and distracted I am. By the time I'm done, I feel calm and relaxed, and I have room in my head and heart to address the Theoi as They should be addressed.
Note that I'm not dirty at all, so technically do not need a washing, but mentally, I'm sullen, distracted. I'm not in the right frame of mind to address the Theoi. If I were to do so without washing, I would be focused on my work, on tomorrow, on the pain in my back, on my exhaustion. After washing, I feel powerful, pious. I feel like the best version of myself, who comes to the Theoi with achievements under her belt, provided by the Theoi. I feel blessed.
It's felt like this for me from the first time I prepared khernips and washed myself with the lustral water.
Personally, I think the importance of katharmos can not be overstated. You can view it as a necessary step you need to take, or as a way to bring yourself closer to the Gods; I think the latter is more constructive.