When the PSI climbs into the Unhealthy range, NEA advises healthy adults to reduce prolonged or strenuous outdoor exertion — which rules out the long run and the hard session. (How to read the numbers: our PSI guide for runners.)

That does not mean a week off. Here are six indoor swaps that keep you fit while the air clears.

1. Treadmill run

The obvious one, and better than its reputation if you set it up properly.

  • Set 1% incline to make up for the lack of air resistance.
  • Do intervals rather than a long steady run. Thirty minutes of 3 minutes steady / 1 minute faster beats 75 minutes of staring at a wall.
  • Turn a fan on you. Treadmills feel hotter than the road because there is no airflow. Feet heat up and sweat faster too, which is when socks start to slide inside the shoe — a sock that stays put matters more here than outdoors. (Why feet slide)

2. Gym intervals: bike, rower, ski erg

Low impact, high effort, and easy to measure.

  • Bike: 6 × 3 minutes hard / 2 minutes easy.
  • Rower: 5 × 500m with 90 seconds rest.
  • Ski erg: 10 × 1 minute on / 1 minute off.

Good for keeping your aerobic engine running without adding pounding to legs that are used to the road.

3. HYROX-style circuit

If you are training for HYROX anyway, haze days are station days.

  • 4 rounds: 500m row, 20 wall balls, 20m sled push, 20 walking lunges.
  • Keep transitions short to keep the heart rate up.

Sled pushes are where feet slip most inside shoes. See feet slipping on the sled if that is you.

4. Pilates or reformer class

A good haze-week choice for runners. Hip, glute and core strength are exactly what long runs lean on, and a reformer class works them without impact. Most Singapore studios ask for grip socks on the carriage.

5. Indoor court sports

Badminton, squash and indoor futsal give you intervals without calling them intervals. Short sprints, quick changes of direction, plenty of lateral movement. (Grip socks for court sports)

6. A proper strength session

Most runners skip strength because there is always a run to do instead. A haze week removes that excuse. Two sessions of squats, lunges, step-ups, calf raises and core work will do more for your next race than two junk miles in bad air. (How often to train each muscle)

Keeping your running fitness

  • Keep one quality session. Make it a treadmill or bike interval session.
  • Keep the total time roughly the same as your normal week, split across indoor sessions.
  • Drop the long run, don't replace it with a 2-hour treadmill slog. A week without a long run costs very little.
  • Come back gently. When the PSI drops back to Moderate, your first outdoor run should be easy, not the long run you missed.

Frequently Asked Questions

Is a treadmill as good as running outside?

For a week, close enough. Use 1% incline and intervals, and keep a fan on you.

Will I lose fitness during a haze week?

Very little, if you keep moving indoors. Fitness fades over weeks, not days.

Are indoor gyms safe during haze?

Air-conditioned gyms with closed doors are usually the easier place to train. Check NEA's advice for the day, and stop if you feel unwell.

What should I wear for indoor training?

Light, breathable kit, and socks that grip inside the shoe or on a studio floor. Gym floors and treadmills get just as sweaty as the road.