Most coverage of London’s air treats it as a league table or a political argument. But for the roughly one in five Londoners living with a lung or heart condition, air quality is a daily, practical question: is today a day to take the usual walk to the shops, or a day to carry the inhaler and cut it short? The answer already exists, published every day, in a number most people have seen but never had explained — the Daily Air Quality Index.
What the Daily Air Quality Index Actually Measures
The Daily Air Quality Index, or DAQI, is the UK’s standard system for communicating short-term air pollution risk. It is not a single pollutant reading; it is built from five pollutants monitored across the country — nitrogen dioxide, ozone, sulphur dioxide, and both coarse and fine particulate matter (PM10 and PM2.5). Each pollutant is measured against its own concentration thresholds, and the index takes whichever pollutant is currently scoring worst at a given location. That matters because it means a single high DAQI reading could be driven by traffic-related nitrogen dioxide on one street and by wildfire or Saharan dust-driven particulates on another, even though the number looks the same.
The index runs from 1 to 10 and is grouped into four bands, each with its own colour and its own health advice. It updates through the day rather than being fixed each morning, which is why the same postcode can move from Low to Moderate between a still morning and a busier, warmer afternoon.
The Four Bands, From Low to Very High
The banding is deliberately simple so it can be understood at a glance, but the practical difference between bands is real, particularly for people whose airways are already reactive.
| Band | Index score | General population advice | Advice if you have asthma or a lung/heart condition |
|---|---|---|---|
| Low | 1–3 | Enjoy usual outdoor activities | Continue as normal, though some people may still notice symptoms |
| Moderate | 4–6 | No change needed for most people | Consider reducing strenuous activity outdoors if you notice symptoms |
| High | 7–9 | Adults with heart or lung conditions should watch for symptoms | Reduce strenuous outdoor exertion; a reliever inhaler may be needed more often |
| Very High | 10 | Reduce strenuous physical exertion generally | Avoid strenuous outdoor activity; older people and children with lung problems should take particular care |
Why the At-Risk Advice Is Different
The DAQI’s health messaging is split deliberately into two tracks: advice for the general population, and separate advice for what public health guidance calls at-risk individuals — principally people with existing asthma, COPD, or heart conditions, along with older adults and children. This split exists because airway inflammation and reduced lung reserve mean the same pollutant concentration produces a measurable response in a sensitised airway well before it produces any symptom at all in a healthy one. A Moderate reading that a fit adult would never notice can be enough to trigger increased inhaler use in someone with poorly controlled asthma.
The consistent recommendation across every band, including Low, is the same: follow whatever management plan you already have agreed with a GP or asthma nurse. If you don’t have one, that is worth arranging before the next High or Very High day arrives, rather than during it. The index is only useful as a planning tool if there is already a plan in place to react to it — on its own, a number on a screen changes nothing.
Reading Your Own Postcode
The published index is not just a single London-wide figure. Because pollution varies block by block — a reading beside a major road corridor can differ meaningfully from one in a nearby park or residential side street — postcode-level checking tools exist precisely so that the number reflects where someone actually is, not a citywide average. Entering a home or work postcode into one of these tools returns the current band for that specific location, along with, on most services, a short forecast for how it is expected to move through the rest of the day. For anyone managing a lung or heart condition, checking before a planned outdoor exertion — a run, a school pickup on foot, a long day outside — takes the same few seconds as checking the weather, and answers a more immediately relevant question.
Building the Index Into a Daily Routine
The DAQI was designed to be acted on quickly, not studied. For most households the useful version of this is small and repeatable:
- Check the current band for your postcode alongside the weather, particularly on still, warm, or heavily trafficked days when pollution tends to build up.
- Know your own or your child’s asthma action plan in advance, so a Moderate or High reading has an obvious response rather than requiring a decision under pressure.
- On High or Very High days, shift strenuous outdoor activity to a different time or a quieter route rather than cancelling it outright where possible.
- Keep reliever medication accessible on days flagged Moderate or above, even if symptoms haven’t started yet.
- Treat a pattern of repeated High readings in one location — a school run, a regular commute — as worth raising with a GP, since chronic low-grade exposure is a different clinical question from a single bad day.
The Index as Population Health Infrastructure
Viewed from a system level rather than a household one, the Daily Air Quality Index is the interpretive layer sitting on top of London’s underlying pollution monitoring network. Raw pollutant concentrations are of limited use to the public without a translation into what they mean for someone deciding whether to go for a run; the banding system exists to do exactly that translation, consistently, every day, everywhere in the country. That is precisely the kind of practical, evidence-based public health tool the wider prevention agenda set out in Better Health for London called for — not a single grand intervention, but the accumulation of small, well-communicated pieces of information that let people manage their own risk. For respiratory and cardiovascular health in a city where outdoor air quality remains a live concern, the index is one of the more immediately usable pieces of that infrastructure, provided people know what the colours actually mean before the day they need them.