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Air quality and opening windows: what the AQI means for ventilation

OpenWindow.live · Updated May 2025

Temperature and humidity get most of the attention in ventilation decisions, but air quality is the third variable that can make opening windows harmful rather than helpful. The Air Quality Index (AQI) measures outdoor pollution from particulate matter, ozone, nitrogen dioxide, and other pollutants. Understanding what the numbers mean for ventilation is straightforward once you know the thresholds.

The AQI scale and what it means for windows

The AQI used by most global monitoring networks runs from 0 to 500. For home ventilation purposes, the relevant thresholds are:

• AQI 0–50 (Good): Open freely. Outdoor air is cleaner than typical indoor air and ventilation is unambiguously beneficial.
• AQI 51–100 (Moderate): Ventilate normally. Sensitive individuals (asthma, heart conditions) may want to limit prolonged exposure, but short burst ventilation is fine for everyone.
• AQI 101–150 (Unhealthy for Sensitive Groups): Shorten ventilation windows. 5–10 minute bursts for CO₂ exchange are still worthwhile, but avoid leaving windows open for hours.
• AQI 151–200 (Unhealthy): Minimise ventilation. Brief bursts only when CO₂ is a concern. Close windows the rest of the time.
• AQI above 200: Keep windows closed. Indoor air, even stale, is better than outdoor air at this pollution level.

When AQI spikes: the main causes

AQI doesn't stay constant through the day or year. The main events to watch for:

• Morning rush hours (7–9 AM): Traffic-related NO₂ and PM2.5 peak in urban areas. This coincides with the temperature window when ventilation is otherwise ideal — check AQI before opening windows in city centres.
• Wildfire smoke: Can raise AQI from 20 to 300+ in hours, even hundreds of kilometres from the fire. Smoke particles are fine enough (PM2.5) to penetrate even closed homes, but open windows dramatically accelerate intake.
• Saharan dust events: Common in southern Europe and North Africa in spring. Dust particles elevate AQI for 1–3 days and settle on surfaces. Close windows during dust events.
• Pollen: Not measured by standard AQI but acts similarly for allergy sufferers. High pollen days may warrant closed windows even when AQI is low.

City patterns: when AQI is typically best

In most cities, AQI follows a daily pattern: it rises during morning rush hour, drops slightly at midday, rises again in late afternoon (5–7 PM), then falls overnight as traffic stops. Late night and early morning hours (11 PM–7 AM) typically have the best urban AQI — which conveniently aligns with the temperature window when night ventilation is most effective.

This coincidence works in favour of passive cooling: the time when outdoor air is coolest is usually also the time when it's cleanest. The main exceptions are industrial areas where night-time emissions differ, and peri-urban areas prone to ground-level ozone formation in hot afternoons.

Indoor air quality vs outdoor air quality

The EPA estimates indoor air is 2–5 times more polluted than outdoor air on average. This includes VOCs from furniture, cleaning products and building materials, plus CO₂ from breathing and particulates from cooking. The comparison shifts when outdoor AQI exceeds 100: at that point, outdoor air can be measurably worse than typical indoor air for particulates specifically, even if indoor CO₂ is elevated.

OpenWindow.live pulls real-time AQI data from the nearest monitoring station for your location and incorporates it into the ventilation recommendation alongside temperature and humidity — so you don't have to cross-reference separate apps.