In short
Sensitive groups feel polluted air from an AQI of about 101, and from 201 upwards the risk rises for everyone, not only people with asthma. A properly sealed N95, filtered indoor air and less outdoor exertion are what work. Cloth masks and lung detox routines do not.
Emergency if: Severe breathlessness at rest, a reliever inhaler that is not working, being too breathless to finish a sentence or, in a child, to feed, chest pain or tightness, confusion or drowsiness, or blue or grey lips, tongue or fingertips: call emergency services now.
On your report
Absolute Eosinophil Count, CRP
On this page
01
AQI 150, 300 and 450: what is happening in the body?
The air quality index compresses several pollutants into one number with six bands. Values of 0 to 50 are good and 51 to 100 moderate, where there may still be a risk for some people, particularly those who are unusually sensitive to air pollution. From 101 to 150 members of sensitive groups may feel effects while the general public is less likely to. From 151 to 200 some of the general public feel effects too. From 201 to 300 the risk is increased for everyone, and 301 and above is a health warning of emergency conditions in which everyone is more likely to be affected.
Notice that the scale stops describing anything new above 301. An index of 450 is not a different category from 320; it means the same warning applied to a heavier and usually longer exposure. It is also worth knowing that the damage is not confined to the lungs. Of the estimated 4.2 million premature deaths attributed to outdoor air pollution worldwide in 2019, about 68 per cent were from stroke and ischaemic heart disease, 14 per cent from chronic obstructive pulmonary disease, 14 per cent from lower respiratory infections and 4 per cent from lung cancer.
02
Do N95 masks work, and why do cloth and surgical masks not?
N95 respirators work, on one condition: they must seal. The EPA advice for smoke is to choose a particulate respirator that has been tested and approved by NIOSH and is printed with the word NIOSH and either N95 or P100. Studies show that when worn according to the manufacturer's instructions, with gaps between the face and the respirator minimised, these significantly reduce exposure to smoke particles.
Cloth masks, surgical masks, bandanas and ordinary dust masks are not designed to seal to the face, and the EPA's guidance on smoke is that dust masks and surgical masks do not prevent smoke inhalation. The failure is mostly at the edges rather than through the fabric: leakage around the rim lets unfiltered air straight in. The EPA is explicit that you should not choose a mask with a single strap or with two straps that loop around the ears, because those cannot seal. The respirator must fit over the nose and under the chin, and the seal should be checked as the packet instructs. Facial hair along the sealing edge defeats it. Two limits are worth knowing before buying a box. Approved respirators are not made in sizes that fit young children, so a sealed respirator is not the plan for a small child; indoor time with filtered air is. And the EPA advises anyone with heart or lung problems, and anyone who is pregnant, to ask a doctor before using a respirator, because breathing through one takes more effort.
03
Air purifiers: which room, how many hours, and does HEPA help?
A portable air cleaner filters a single room, which decides how to use one sensibly. The bedroom with the door closed, running for the seven or eight hours you sleep, is usually the best value from one machine, because that is the longest continuous block anyone spends in one place. Match the unit to the room size, run it continuously rather than in bursts, replace the filter on schedule, and avoid ozone-generating devices. The EPA is clear that filtration is a supplement to controlling sources and ventilating, not a replacement for them.
On health outcomes the honest answer is mixed. A meta-analysis of 14 randomised trials in more than 700 participants across four countries found that air purification cut indoor fine particles by an average of 56 per cent and lowered systolic blood pressure by 2.28 mmHg, with no significant change in diastolic pressure and no clear change in CRP, interleukin-6 or fibrinogen. The authors graded the certainty of the evidence as very low and asked for caution in claiming benefits. So: reliably cleaner air, a modest and uncertain health gain.
04
Do children and people with asthma need different rules?
Yes, and the index itself says so. Sensitive groups begin to feel effects in the 101 to 150 band, one full step before the general public does. Air pollution is described by WHO as one of the greatest environmental risks to child health, and children are more exposed for practical reasons: they breathe more air relative to body size, spend more time outdoors, and are less able to say when something feels wrong.
If you or your child has asthma or chronic lung disease, smog season is the wrong time to reduce the preventer inhaler. Keep the reliever inhaler physically with you rather than at home, know what your written action plan says to do when symptoms step up, and treat rising reliever use as the early warning it is. Rescheduling sport indoors for a few weeks costs little; an avoidable attack during a pollution spike costs a great deal more.
05
Should you stop the morning walk?
Move it rather than abandon it. The EPA's most effective single measure is to limit time outdoors when there is smoke in the air, with reduced physical activity and indoor air cleaners as the other main ways to cut exposure. Exercise works against you here for a mechanical reason: harder breathing draws far more air per minute, and therefore far more particles, into the lungs than sitting quietly does.
The practical version is to check the local reading for the hour rather than the daily headline figure, and pick the cleanest window rather than a habitual time. On days in the hazardous band, take the session indoors in a room you are filtering. On moderate days, keep walking, though if you have asthma, another chronic lung condition or heart disease, treat the sensitive-group line at 101 as your threshold rather than waiting for the number that applies to everyone else. The cardiovascular benefit of regular exercise is not something to give up for a whole season, and a plan that says never go out again is a plan nobody follows.
06
Steam, herbal drinks and lung detox: what does the evidence say?
What works is unglamorous: spend less time in the polluted air, filter the air where you sleep and work, and wear a properly sealed NIOSH-approved respirator when you must be out in it. Promising but modest is the portable HEPA purifier, which reliably lowers indoor fine particles by around half and shows a small blood pressure benefit on very low certainty evidence.
What to skip is anything sold as clearing the lungs after the fact. There is no evidence that steam inhalation, herbal drinks or lung detox regimens remove inhaled particles, and none of them appear among the protective measures recommended by WHO, the EPA or the CDC. The lung clears particles through its own mechanisms on its own timescale. Steam also scalds, particularly children. If a warm drink eases a scratchy throat, there is no harm in it, as long as it does not take the place of a mask or a filter.
07
What are you adding to your own indoor air?
Indoor sources can dwarf what comes in through the window. Around 2.1 billion people cook using polluting fuels and inefficient stoves, and household air pollution was responsible for an estimated 2.9 million deaths in 2021, including more than 309,000 children under five. The problem fuels are wood, crop waste, charcoal, coal, dung and kerosene; the cleaner options are electricity, liquefied petroleum gas, natural gas, biogas, solar and alcohol fuels. Women and children, who spend the most time near the stove, carry most of the burden.
Mosquito coils deserve a specific mention. Measurements found that burning one coil releases the same mass of fine particles as burning 75 to 137 cigarettes, and the formaldehyde from one coil can match that from 51 cigarettes. Concentrations from indoor burning can substantially exceed health-based air quality guidelines. Nets, screens and plug-in devices avoid the smoke entirely. The same logic applies to incense and to smoking indoors: the EPA's first instruction is to reduce or remove the sources before relying on any filter.
08
Which symptoms need a doctor?
A dry cough, a scratchy throat, itchy eyes and a heavy feeling in the chest during a smog spell are common, and they settle as the air improves. What is not routine is breathlessness at rest or on minimal effort, wheeze that the usual inhaler does not settle, chest pain or tightness, coughing up blood, fever with breathlessness, which points more towards an infection than towards the air, or symptoms that persist well after the air has cleared.
There is no blood test that measures how much pollution you have breathed. Counts such as the absolute eosinophil count or CRP are used to work out whether asthma or an infection is driving your symptoms, not to quantify exposure, so a normal result does not mean the air is harmless and an abnormal one does not mean the air caused it. If breathlessness persists, the useful investigation is a lung function test rather than another round of blood work.
When smog symptoms need a doctor
Routine — see a doctor
Cough, sore throat or itchy eyes during a pollution spell that ease once the air improves: reduce exposure, filter the room you sleep in, and see a doctor if symptoms outlast a week of cleaner air.
Same-day — call promptly
Needing your reliever inhaler far more often than usual, wheeze or a night cough that settles with the reliever but keeps returning, or fever with breathlessness: get a same-day appointment.
Emergency — act now
Severe breathlessness at rest, a reliever inhaler that is not working, being too breathless to finish a sentence or, in a child, to feed, chest pain or tightness, confusion or drowsiness, or blue or grey lips, tongue or fingertips: call emergency services now.
Values mentioned in this story
Each one opens a visual guide with its normal range and what moves it — upload a report and we place your own numbers on the same scale.
Sources
- WHO: Ambient (outdoor) air quality and healthwho.int
- AirNow: AQI Basicsairnow.gov
- US EPA: Protect Yourself From Wildfire Smoke and Ashepa.gov
- Indoor air purification and cardiovascular health: systematic review and meta-analysis of RCTspmc.ncbi.nlm.nih.gov
- Mosquito coil emissions and health implicationspmc.ncbi.nlm.nih.gov
- WHO: Household air pollution and healthwho.int
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