Smoke, secondhand smoke, traffic pollution, pollen and household dust — that short list covers the overwhelming majority of what a normal set of lungs has to filter. None of it is exotic. All of it is worth understanding properly, because what you do about each one differs.
Pollen grains are large as airborne particles go, which means most are caught early: nasal hairs, the upper airway, the mucus blanket. The trouble isn't that they reach deep tissue. It's that the immune system of a sensitised person reacts vigorously to them anyway, producing the streaming, itching, congested response that defines hay fever.
The itch is the diagnostic clue. Colds don't generally itch. A "cold" lasting six weeks in April is almost never a cold.
Reduces exposure: showering and changing after long spells outdoors, keeping bedroom windows shut during peak counts, drying laundry indoors in season, and car recirculation on.
Everything else on this list gives your defences more work. Tobacco smoke takes the defences away. The cilia lining your airways stop beating under it, and with enough years they are lost altogether. Strip out the mechanism that normally carries mucus upward and only one option remains for shifting anything — which is the entire reason a smoker's cough is a recognisable thing.
Secondhand smoke matters for the same reasons at lower intensity, and it matters most in enclosed spaces — cars especially, where concentrations get high fast.
Wildfire smoke is a separate category: dense PM2.5, small enough to reach the alveoli where there's no escalator to sweep it out, and capable of travelling hundreds of miles.
Reduces exposure: for tobacco, stopping — it outranks everything else available to you, at any age. For wildfire days, closed windows, recirculated air conditioning, a correctly sized HEPA unit in the room you actually occupy, and postponing outdoor exercise rather than pushing through.
Roadside air carries nitrogen dioxide and fine particulate, and concentrations fall off quite sharply with distance from the carriageway. That geography is the useful part: a parallel back street a block over is measurably better air than the main road, and cycling or walking one street in is a real reduction rather than a token one.
Inside a car in traffic is not the refuge people assume, since you're sitting in the exhaust plume of the vehicle ahead. Recirculation helps.
Dust isn't inert fluff. It carries dust mite allergen from bedding and soft furnishings, pet dander, fragments of textile fibre, and whatever has been tracked in on shoes. For a large number of people, the dust mite component is the single most relevant year-round allergen in their life, and the bedroom is where the exposure concentrates because that's where you spend eight unmoving hours.
Reduces exposure: weekly hot washing of bedding, a vacuum with a sealed HEPA system rather than one that redistributes fines, keeping humidity under 50 percent since mites need moisture, and shoes off at the door.
More than most people realise, and none of it requires your attention. Incoming air is filtered, warmed and moistened on the way through the nose. Whatever slips past meets a sticky layer built to hold it. Beneath that layer, cilia move in rolling waves and carry the whole lot up to the throat, where you swallow it without ever knowing. The everyday load is well inside what that arrangement copes with. It struggles under long stretches of heavy concentration, and it is dismantled outright by smoking.
Which is the reason any product promising to cleanse or flush a lung falls apart on inspection. Clearance is already running, continuously, and a liquid you drink travels to your stomach rather than across the inside of a bronchus. What a daily botanical habit can honestly offer is far more modest — a contribution to healthy airway function, sitting underneath the measures listed above and not in place of a single one of them.