Chicago, Illinois, United States

HOW THE AIR GETS THERE

You move roughly 11,000 litres of air through your lungs a day, and every particle in it gets a chance to land. Where it lands is decided by size: PM10 is mostly caught in the nose and throat, PM2.5 reaches the bronchioles, and the finest fraction settles in the alveoli — the 300 million air sacs where the barrier to your bloodstream is a single cell thick. Reactive gases work differently: ozone does not need to be deposited, it chemically oxidises the airway lining on contact, which is why it hurts to breathe deeply on a high-ozone afternoon.

WHAT TO EXPECT AT TODAY'S LEVEL

Nothing expected here — this is clean air, and a good day to be out in it.

HOW TO PROTECT IT

• A well-fitted N95/FFP2 is the only mask that meaningfully filters PM2.5 — cloth and surgical masks do very little, and fit matters more than the rating printed on it
• Breathe through your nose when you can: it filters and humidifies air that mouth breathing sends straight down
• Lower intensity rather than cutting duration — ventilation rises steeply with effort, so an easy hour costs your lungs far less than a hard twenty minutes
• Run a HEPA purifier in the bedroom; eight hours of clean air a night is the single biggest reduction in daily dose available to most people
• Do not add indoor smoke on a bad day — no frying without extraction, no candles or incense, and vacuum only with a HEPA filter
• If you use a preventer inhaler, take it as prescribed; it is what gives your airways margin when the air is poor

OVER YEARS

Years of exposure accelerate the natural decline in lung function, and in children stunt lung growth so they never reach full capacity — a deficit that does not recover in adulthood. It causes chronic bronchitis and COPD, and outdoor particulate pollution is classified by IARC as a Group 1 human carcinogen, causing lung cancer in people who have never smoked. Traffic-related NO₂ has the strongest evidence of any pollutant for causing new-onset asthma in children.

HOW THE AIR GETS THERE

Your cornea and conjunctiva sit in the airstream with nothing between them and the atmosphere. Particles land on and abrade the tear film — the thin layer that keeps the surface smooth and optically clear — while reactive gases such as ozone, sulphur dioxide and formaldehyde dissolve straight into it. Because the tear film is your first defence and also your outermost lens, damaging it causes both irritation and genuinely blurred vision. Contact lenses make this markedly worse: they destabilise the tear film, hold deposits against the cornea and reduce the flushing effect of blinking.

WHAT TO EXPECT AT TODAY'S LEVEL

Nothing expected here — this is clean air, and a good day to be out in it.

HOW TO PROTECT IT

• Switch from contact lenses to glasses on poor-air days — glasses also act as a partial physical shield
• Do not rub. Flush instead: clean water or preservative-free saline, from the inner corner outwards
• Preservative-free artificial tears restore the tear film and physically wash irritants away; the preservatives in cheaper drops can add to the irritation
• Wraparound sunglasses on dusty or windy days block a surprising amount of particulate from reaching the surface
• Blink deliberately when on screens in polluted air — reduced blink rate and a damaged tear film compound each other
• See a clinician for pain, light sensitivity or vision that stays blurred after flushing, rather than waiting it out

OVER YEARS

Sustained exposure is associated with chronic dry eye disease and chronic conjunctivitis, and studies in high-pollution cities find measurably higher rates of both. There is growing evidence linking long-term particulate exposure to glaucoma and to age-related macular degeneration, likely through the same oxidative stress and microvascular damage that drives its effects elsewhere in the body.