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When a self-built air purifier outperforms a £220 commercial unit in rigorous home testing, it forces a reckoning with how we measure air quality effectiveness. A recent experiment by physics enthusiast 'CasualPhysicsEnjoyer' pitted a DIY Corsi-Rosenthal (CR) box against a Levoit 400s purifier and natural ventilation, revealing surprising performance gaps and unsettling ambiguities in the industry's gold-standard metric: Clean Air Delivery Rate (CADR).

The Kale-Fueled Experiment

The tester designed a repeatable pollution scenario: sealing a room, frying 120g of kale in 30g of oil to generate consistent PM2.5 levels (~450 μg/m³), then measuring decay rates under three conditions:
- Natural ventilation (open window)
- Commercial Levoit 400s (claimed 400 m³/h CADR)
- DIY CR box (four MERV-13 filters + box fan)

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The DIY Corsi-Rosenthal box used in testing (Credit: CasualPhysicsEnjoyer/Substack)

Using a PM2.5 monitor, decay rates were logged and converted to CADR using logarithmic regression. The DIY unit achieved 550 m³/h—37% higher than the Levoit's specification. Natural ventilation trailed significantly. The CR box's victory, however, unraveled into a deeper critique of CADR itself.

The CADR Conundrum

Three critical flaws emerged during analysis:
1. Room Mixing Myths: "Measuring at a single point assumes perfect air mixing," noted the experimenter. Without multiple sensors or forced circulation, decay rates could reflect pollutant dispersion rather than true filtration.

  1. Volume Vagaries: CADR calculations require precise room volume measurements—nearly impossible in irregular spaces. The tester used LiDAR scanning (via MagicPlan app), but acknowledged inherent error margins.
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PM2.5 decay rates: CR box (red) vs. natural ventilation (blue) (Credit: CasualPhysicsEnjoyer/Substack)

  1. Pollutant Paradox: Commercial CADR tests rarely disclose pollutant types. Kale smoke (PM2.5) behaves differently than viruses or pollen, making cross-comparisons unreliable. "You can't expect CADR measured from burning vegetables to match pathogen removal," the report emphasized.

The Practical Compromise

Despite outperforming on paper, the CR box has real-world limitations:
- Noise pollution limits high-speed use
- No pathogen efficacy data
- Aesthetic and space constraints

Commercial units compensate with quieter operation and continuous runtime—factors CADR ignores. This highlights a critical industry gap: standardized real-world testing protocols.

Toward Better Air Metrics

The experimenter plans further tests with multi-sensor Arduino arrays and match smoke (more standardized than kale). This DIY approach underscores a broader truth: when commercial specifications lack transparency, independent verification becomes essential. As airborne health risks escalate, this investigation proves we need more than marketing metrics—we need reproducible, real-world validation.

Source: CasualPhysicsEnjoyer/Substack