What speakers mean by it now
In current English a purifier is whoever or whatever strips out what does not belong. Most people meet the word as hardware: a fan that pulls dust and pollen from a room, a cartridge under a sink, a filter on a laboratory line. Dictionaries still keep a wider sense beside that picture. The noun can name a person who cleanses, or a substance used for the same work. It is an agent word. Purity is the state. Purification is the act. The purifier is the doer.
That split between act and agent is the whole of the morphology. English did not borrow a ready-made machine name from Latin. It took a verb and then, later, hung a native ending on it.
A French verb finished with an English ending
The origin of the word purifier is that join. The noun is purify plus the agent suffix -er, the same Germanic ending that turns bake into baker and grind into grinder. The verb is not Germanic. Middle English adopted it from Old French purifier, also written purefier, a twelfth-century French loan from Latin pūrificāre, to make pure.
Dating is the one point where the references part company. For the verb, some dictionaries put the English borrowing in the years 1250 to 1300; others first record Middle English purifien in the mid-fourteenth century. Those later notices often give an early spiritual sense, freeing a person from stain, and a physical sense, stripping extraneous matter from a substance, only toward the end of the same century. Anglo-Norman already used the French verb of water and of sin around 1300, so the English dates are a matter of surviving manuscripts, not of a sudden invention. For the noun, one standard etymological account first finds it in the 1550s as a person who purifies. Another labels the formation Middle English without printing a year. The cautious reading is that the verb is medieval and that the agent noun is securely in print by the middle of the sixteenth century.
Romance languages often built a different agent on the same Latin stem: French purificateur, Italian purificatore, Spanish purificador, Portuguese purificador. Those forms follow a Latin type in -ātor. English kept the French-looking verb and attached its own -er instead of importing the longer Latin agent. The result looks native at the end and borrowed at the root, which is exactly how it was made.
Two Latin pieces, and a reconstructed root
Latin pūrificāre is transparent once it is split. The first half is pūrus, clean, unmixed, unstained. The second is the verb-forming piece -ficō, from facere, to make, the same element English hears in -ify. A purifier, taken apart, is a maker of purity.
The Latin verb does not belong to the oldest layer of the language. Classical lexica mark pūrificō as post-Augustan. Pliny uses it of washing muddy feet with water; Suetonius uses it of ritual cleansing. A few modern English dictionaries call the source Late Latin, which stretches the label. The verb is Imperial, already in use in the first and second centuries, not a medieval coinage. Older Latin had other verbs for similar work, above all purgāre, the ancestor of English purge. Purify and purge are cousins, not twins.
Behind pūrus, comparativists reconstruct a Proto-Indo-European root written *pewH- in recent handbooks and *peue- in older ones, with a sense of being clean or of cleansing. That form is a reconstruction, not a word anyone wrote down. Related descendants include a Sanskrit verb used of purifying, Sanskrit pūtá for what has been made clean, Latin putus for something clear, Middle Irish úr for what is fresh, and Old High German fowen, to sift. Facere goes back to a different root, *dʰeh₁-, to set or put. The English noun therefore sits on two Indo-European pieces glued together in Latin, passed through French, and finished with a Germanic suffix.
Latin pūrus had already reached English as the adjective pure, from Old French pur, by the thirteenth century. Purify and its agent noun are later members of that family, not a separate idea borrowed from nowhere. Native Old English had hlūtor for pure; the French-Latin set largely took over this corner of the vocabulary.
From a person to a plant that scrubbed gas
The first English sense of the noun is personal. A purifier was someone who made people, places, or materials clean, often inside a religious or moral frame, because that is how the verb had arrived. Latin already mixed the two jobs, washing dirt and performing rites. Medieval French and English kept both. Anglo-Norman even used the verb for the churching of women after childbirth, a liturgical cleansing forty days after delivery.
The industrial sense is much later. By 1834 the word names a mechanical apparatus, first in connection with coal gas: equipment that stripped sulphur compounds and tar so the gas would burn in lamps and streets. Nineteenth-century definitions also apply it to mill machinery that separates fine bran from flour middlings. The household air unit and the kitchen water filter are the same agent pattern aimed at newer pollutants. The personal sense has not been deleted from the dictionaries, but everyday use has moved toward devices.
A nearby church word is worth keeping distinct. Purificator, recorded by the mid-nineteenth century for the linen used to wipe a chalice at Mass, is a separate agent noun formed on the Latin verb. It is not a respelling of purifier, and it never became the name of a machine.
Cognates of the verb stay close across Romance: French purifier, Italian purificare, Catalan and Portuguese purificar, Romanian purifica. Languages that did not take this compound use other roots for the device: German Reiniger, Polish oczyszczacz, Swedish renare. English relatives that share the Latin stem include purity, purification, and Puritan. Purge, as noted, comes from the neighboring Latin verb, not from pūrificāre itself.
The ending that waited for industry
The Latin chemistry of pure-plus-make is not the surprise. The surprise is how long the English agent stayed a person. For roughly three centuries after the noun is first recorded, a purifier is still someone who cleanses. Only when coal gas needed scrubbing did the old -er settle on iron and brick. The suffix had been ready the whole time. Industry gave it something new to attach to.