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The short answer
Run it all day and all night, on the lowest speed that keeps the room turning over, with the display off. Boost to a higher speed for 30–60 minutes after cooking, after cleaning, after anyone opens a window, and after a nappy change.
The reason is mechanical. A purifier does not clean a room in one pass. It reduces the concentration of particles gradually, and the concentration only stays low while the unit keeps running. Switch it off and the room drifts back toward its baseline within a couple of hours.
Why continuous beats intermittent
| Approach | What happens | Result |
|---|---|---|
| Continuous on low | Steady 4–5 air changes per hour all night | Low particle concentration maintained |
| Intermittent on high | Big drop, then a slow climb back | Average concentration higher, and noisy while it runs |
| Night only | Good while sleeping, poor during the day | Better than nothing; misses cooking and dust peaks |
| Off overnight | Nothing during the eight hours that matter most | The worst option for a nursery |
There is also a comfort argument. A fan pinned to one low speed is far less likely to wake a light sleeper than a fan that ramps up and down. That alone is why sleep mode beats auto mode overnight — covered in more depth in our quiet-nursery pillar.
The air-changes maths
Air changes per hour (ACH) is the number of times the unit moves the room's whole air volume through its filter in an hour. For a nursery, four to five is a sensible target.
Required CADR (CFM) = room volume in ft³ × target ACH ÷ 60
Worked example for an 8 × 10 ft nursery with an 8 ft ceiling:
- Volume = 8 × 10 × 8 = 640 ft³
- At 5 ACH: 640 × 5 ÷ 60 = about 53 CFM
- At 4 ACH: 640 × 4 ÷ 60 = about 43 CFM
So a unit with a smoke CADR in the 50s holds that room at five air changes on whatever speed produces that figure — usually the lowest or second-lowest, which is where the quiet is. The full sizing walkthrough, including cot corners in studios, is in our nursery sizing guide.
Working out your runtime
| Step | What to do | What it tells you |
|---|---|---|
| 1 | Measure room volume (L × W × H in ft) | The air the unit has to turn over |
| 2 | Multiply by 4–5 ACH and divide by 60 | The CFM you need |
| 3 | Compare with your unit's smoke CADR | Whether low speed alone is enough |
| 4 | If CADR is higher, stay on low all day | You can run 24/7 quietly |
| 5 | If CADR is lower, boost for 1 hour, twice a day | Compensate without running turbo overnight |
What it costs to run
This is the reassuring part. Purifiers are low-draw appliances, and on low speed the draw is very low.
| Speed | Typical power draw | Hours per day | Monthly kWh | Monthly cost (at $0.17/kWh) |
|---|---|---|---|---|
| Low / sleep | 2–5 W | 24 | 1.4–3.6 | About $0.24–$0.61 |
| Medium | 15–30 W | 2 (boosts) | 0.9–1.8 | About $0.15–$0.31 |
| High / turbo | 40–60 W | 1 | 1.2–1.8 | About $0.20–$0.31 |
| Whole month, mixed use | — | — | Around 3–7 | Roughly $0.50–$1.20 |
Figures are estimates based on typical fan wattages; check your unit's label and your own electricity rate. Energy Star listing is a useful shortcut — a unit that qualifies tends to sit near the bottom of the range above.
The practical conclusion: at roughly a dollar a month, runtime is not the variable worth economising on. Filter replacement will cost you ten to thirty times more over a year than the electricity will.
A practical day-by-day routine
| Time | Mode | Why |
|---|---|---|
| Overnight (all night) | Low / sleep, display off | Steady low-speed filtration while the baby sleeps |
| Morning | Low — boost 30 min if a window was opened | Clears the pollen or street dust a window brings in |
| After cooking | Boost 30–60 min | Cooking generates fine particles and odours |
| After cleaning or vacuuming | Boost 30–60 min | Vacuuming resuspends settled dust |
| Nappy change | Low — boost if needed | Short odour load; carbon handles the rest |
| Before bed | Return to low / sleep | Set it and leave it for the night |
Runtime scenarios compared
| Scenario | Air changes per day | Noise exposure | Filter wear | Verdict |
|---|---|---|---|---|
| 24/7 on low | 96–120 ACH total | Minimal | Normal | Recommended |
| Off overnight, on all day | 48–60 | Low | Slightly less | Misses the key hours |
| 8 hours a day on medium | 32–40 | Moderate | Similar | Inferior to low all day |
| Intermittent on turbo | Variable, high peaks | High | Accelerated | Worst of both |
Runtime and filter life
Continuous running consumes filter media faster, and that is the honest trade for running 24/7. The good news is that most nursery units have a washable pre-filter, which absorbs most of the load.
| Runtime pattern | Pre-filter rinse | HEPA replacement |
|---|---|---|
| 24/7 on low | Monthly | Every 6–9 months |
| 12 hours a day | Monthly | Every 9–12 months |
| Intermittent | Every 2 months | Every 12–18 months |
Change on schedule rather than on smell. A saturated filter stops cleaning effectively and, in a damp room, can begin to re-release what it captured. Our filter replacement guide has the full maintenance calendar.
Quiet Unit Built for Continuous Running
A true-HEPA unit with a low-speed draw in the 2–5 W band and a night mode designed for all-night operation, so 24/7 running costs pennies and stays inaudible at the cot.
Check Price on Amazon →Where to go next
- Where to put an air purifier in a nursery — the placement method.
- What size air purifier for a nursery — the sizing maths in full.
- Best air purifier for a nursery — the shortlist.
- How often to replace an air purifier filter — maintenance cadence.
- Air purifier running cost — the electricity side in depth.
- Quiet air purifier for a nursery — the pillar guide for a silent overnight unit.
- Air purifier noise levels explained — what each speed sounds like.
Sources: AHAM (CADR certification program; two-thirds rule), U.S. EPA (Residential Air Cleaners guide), ASHRAE Standard 62.2 (residential ventilation), DOE / Energy Star (appliance energy use), WHO Environmental Noise Guidelines (night-time bedroom levels).
Frequently asked questions
How long should I run an air purifier in a nursery?
Is it safe to run an air purifier 24/7 in a baby's room?
How much does it cost to run an air purifier 24/7?
Should I turn my air purifier off at night?
How many air changes per hour does a nursery need?
Does running it on high clean the air faster?
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