Atmos Nairobi

Run Cleaner: when Nairobi's air is kindest to runners

Nairobi runners: the air you train in varies 2x by hour. Cleanest window: late morning to late afternoon. Worst: the 7-9am rush and 10pm-2am. A runner breathes 5-10x more air than someone resting, so timing is free protection.

  1. Midday is the trough: boundary-layer mixing plus low traffic puts 11:00-17:00 around 19-21 µg/m³ (still above the WHO daily guideline of 15, but the best on offer).
  2. The morning rush is real: 07:00-09:00 readings ran 42-53 in our sample (fewer sensors reporting overnight, so treat as indicative), which is exactly when most training groups meet.
  3. Nights are dirtier than traffic explains: 22:00-02:00 holds around 36 while roads are empty; overnight burning plus temperature inversion. Dawn runs inherit some of it.
  4. Route matters as much as hour: our sites span 12 (Parklands) to 34 (Manyatta Ruai) weekly means. Forest edges and leafy corridors beat matatu corridors by roughly half the dose.
  5. Dose scales with breathing: a runner ventilates 5-10x resting volume, so an hour's hard run in 40 µg/m³ is a meaningful exposure. Timing is free protection.

Caveats: hourly means pooled across sites; early-morning sample thin; two weeks of data, updating monthly; guideline reference is WHO 24-hour PM2.5 (15 µg/m³). Based on two weeks of hourly public sensor data (AirQo network, 21 Nairobi sites), refreshed monthly.