When should I go outside today?

Hourly air quality and UV

How scores work

Score bands

Great85–100Excellent conditions — go outside
Good70–84Generally good with minor concerns
Fair50–69Acceptable — limit prolonged exposure
Caution30–49Elevated risk — keep outings short
Avoid0–29Poor conditions — stay inside if you can

Activity modes

Each mode weights the three factors differently. Switch modes to see how the score changes for your activity.

Factor General Exercise Sensitive Air quality 40% 45% 55% UV Index 35% 33% 28% Temperature 25% 22% 17%

Exercise mode weights air quality more heavily — you breathe harder, so you're more exposed. Sensitive health mode makes air quality dominant and applies stricter thresholds throughout. Rain and wind aren't scored — they're a matter of preference, not health risk, and are shown separately in the conditions section.

Air quality (US AQI)

0–50Clean air — full score
51–100Moderate — score drops toward 50
101–150Unhealthy for sensitive groups — score drops to 20; Sensitive mode capped at Caution
151–200Unhealthy — score 0; all modes capped at Caution
200+Very unhealthy to hazardous — avoid outdoor activity

UV Index

0–2 LowFull score — no action needed
3–5 ModerateScore 80–60 — consider sunscreen
6–7 HighScore 60–40 — sunscreen essential
8–10 Very highScore 40–10 — limit midday exposure
11+ ExtremeScore near 0 — all modes capped at Caution

Temperature

Score peaks between 10–25°C. It drops gradually in cold conditions and more steeply above 30°C, reflecting the risk of heat stress. Very cold (below −10°C) or very hot (above 35°C) conditions score poorly regardless of air quality or UV.

Find the best time to go outside today

Air quality, UV and weather combined into a single hourly score for your location.

or try an example city

How do I read the air quality numbers?

The US AQI scale runs from 0 to 500, divided into colour-coded bands. 0–50 (good) is clean air; 51–100 (moderate) brings mild effects for the most sensitive; 101–150 affects sensitive groups — people with asthma or heart conditions, young children, older adults, and pregnant people; 151+ is unhealthy for everyone. The score on this page weights air quality most heavily because the pollutants behind these numbers — fine particulate matter (PM2.5), ground-level ozone, and nitrogen dioxide — have measurable short-term and long-term effects on the lungs and the cardiovascular system.

PM2.5 matters more than PM10 of equal mass because fine particles reach the alveoli and cross into the bloodstream. Ozone forms in sunlight from vehicle and industrial emissions, which is why it typically peaks in mid-to-late afternoon on warm days. NO2 concentrates near busy roads. None of these are visible — the colour of the sky is a poor proxy for the score.

Masks, purifiers, route choice, and why countries disagree on AQI

On bad-AQI days, indoor air matters as much as outdoor. A HEPA-filter air purifier sized to room volume is the standard mitigation; closing windows and running it for an hour drops indoor PM2.5 substantially. A well-fitted N95-style respirator reduces PM2.5 exposure by 90% or more for short outdoor trips — surgical masks help less, cloth masks help only marginally. If you exercise outdoors regularly, the simplest behavioural change is route choice: PM2.5 and NO2 drop by 50% or more a hundred metres away from a busy road.

Different countries use different AQI scales. The score on this page uses the US EPA AQI because its health bands are widely cited and well-validated. The UK's Daily Air Quality Index (DAQI) is a 1–10 scale; the EU Common Air Quality Index (CAQI) tops out at 100 with different thresholds. The underlying pollutant readings come from the same satellite and ground-station network — only the framing differs.

Source: US EPA AirNow — AQI basics.

What does the UV index mean?

The UV index runs from 0 upwards, with no upper bound — values above 11 are routine in tropical summer and at altitude. 0–2 needs no action; 3–5 (moderate) is where sun protection starts to matter for fair-skinned people around midday; 6–7 (high) means sunscreen, hat, and shade between roughly 10am and 4pm; 8–10 (very high) can cause sunburn in 15–25 minutes on unprotected pale skin; 11+ (extreme) burns in under 10 minutes.

Burn time varies sharply by skin type. The Fitzpatrick scale (I to VI) is the medical reference: type I (very pale, always burns) and type VI (deep brown to black, never burns visibly) sit at opposite ends of a roughly 4–8× difference in safe sun exposure time. Skin colour does not eliminate UV risk — skin cancer affects all skin types, and high UV still causes deep tissue damage and photoaging across the spectrum.

UVA vs UVB, what SPF actually buys you, and amplifiers

UVA penetrates deeper into the skin and is responsible for photoaging and the bulk of melanoma risk; UVB causes the immediate burn and most non-melanoma skin cancers. Both reach the surface in roughly constant proportion through the day, which is why broad-spectrum sunscreen — filtering both wavelengths — matters more than the SPF number alone. SPF 30 blocks about 97% of UVB; SPF 50 blocks about 98%. The real-world difference is dominated by application thickness and reapplication frequency, not the rating. Standard guidance is to apply about 30 ml (a shot glass) for full-body coverage and reapply every two hours, after swimming, and after heavy sweating.

Sunscreen is the most-discussed defence but rarely the best on its own. UPF-rated clothing blocks UV at the fabric level and does not wear off. A wide-brimmed hat blocks the face, ears, and back of neck where many skin cancers form. Wraparound UV-blocking sunglasses protect the eyes and the skin around them. Conditions amplify exposure in ways that catch people out: UV is roughly 5% higher per 1000 m of altitude, reflects 80% off fresh snow, and 30% off open water. Cloud cover often blocks only 20–30% of UV, not all of it.

Source: NHS — sunscreen and sun safety.

Why does timing matter more than the daily summary?

A single day can have a great morning and a dangerous afternoon. UV peaks between 10am and 4pm — roughly 80% of the day's UV exposure happens in that six-hour window, regardless of how high the peak is. Ground-level ozone forms when sunlight reacts with emissions, so air quality is often worse at 5pm than at 9am even when the headline AQI for the day looks the same. Traffic-related pollutants — NO2 and combustion-related PM2.5 — peak at morning and evening rush hours, especially within a few hundred metres of major roads.

Pollen has its own daily rhythm. Grass and tree pollens peak in the early to mid-morning during the active season; some weed pollens peak in the late afternoon. Hay-fever sufferers often find the same day brutal at dawn and tolerable at lunchtime. The hourly chart at the top of this page combines air quality, UV, and weather into an hour-by-hour score, which is more useful than the daily summary when the goal is finding a window.

Best windows, ozone after sunset, and humidity heat stress

In most temperate cities, the most defensible windows for outdoor exercise are before 9am and after 7pm — earlier in summer, more flexibility in winter. Mid-morning often wins on the air-quality axis but loses on UV; late afternoon wins on UV but loses on ozone. Early morning typically wins on both, with the trade-off being cold and low light. Late evening is the second-best window in summer once UV has dropped and traffic has cleared, though residual ozone can linger past sunset on hot days.

Heat stress compounds with humidity. The body cools by evaporating sweat; high humidity prevents that. Effective temperature climbs steeply above 30°C when humidity is above 70% — heat-related hospital admissions track this combined effect rather than raw temperature. On hot days, time outdoor exertion early or late, carry more water than feels necessary, and add electrolytes for sessions over an hour. Light, light-coloured layers help in dry heat; loose-fitting moisture-wicking fabrics help in humid heat.

Source: US EPA — ground-level ozone basics.

How this guide works

The Go Outside Guide combines three hourly forecasts — air quality, UV index, and weather — into a single score for each hour of today, so you can find your best window without flicking between three different apps. All data comes from Open-Meteo, which aggregates national weather services and the Copernicus air quality monitoring system.

Three modes are available. General is a balanced score for everyday outdoor time. Exercise weights air quality more heavily, because heavy breathing increases exposure. Sensitive health applies stricter thresholds across the board for people with respiratory conditions, young children, or anyone advised to be cautious about pollution and UV.

The recommendation is for general guidance. It does not replace local forecasts, official air quality alerts, or medical advice — particularly if you have a respiratory condition, are pregnant, or are caring for very young or older people.

How is "best window" calculated?

Each hour gets a score from 0–100 based on air quality (US AQI), UV index, and temperature, with weights that depend on the active mode. The "best window" highlights the longest run of consecutive hours that score above the chosen threshold for the mode.

Why does the score sometimes change without the conditions changing?

Forecasts update through the day. Open-Meteo refreshes its model output regularly, and the guide picks up new data on each cache cycle (about once an hour). Small changes in ozone, PM2.5, or cloud cover can shift the score even if the headline weather looks the same.

Which air quality scale is used?

The score uses the US EPA AQI as a common reference because it has clear health bands. Where local agencies use a different scale (for example the UK's Daily Air Quality Index), values may not line up exactly — but the underlying pollutant readings come from the same satellite and ground-station network.

Why do you ask for my location?

Forecasts are local. The guide either uses your browser's geolocation if you grant permission, or geocodes a place name you type. Coordinates are sent to Open-Meteo to fetch a forecast and not stored on dataretold's servers — see the privacy policy for details.

Does it work offline or for past days?

No. The guide is forward-looking and needs an internet connection to fetch live forecast data. Historical air quality and weather lookups are not part of this tool.

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