Cabin Bioaerosol Control: HEPA Filtration Loop

Bioaerosol Source

Crew size 4 astronauts

Air Handling

Air exchange rate 20 ACH

HEPA filter efficiency 99.97%

Cabin volume 120 m³

Live Cabin Air Quality

Airborne bioburden N(t)

— CFU/m³

Steady-state N∞

—

Time constant τ

—

How it works

The cabin is modeled as a single well-mixed compartment. Crew members continuously shed microbial bioaerosols (skin flora, respiration, waste handling) while the environmental control loop pulls cabin air through a HEPA filter and returns it. The airborne concentration N (CFU/m³) obeys a linear clearance ODE:

dN/dt = G − λ·N
λ = (Q/V)·η,   Q = ACH·V / 60
N∞ = G / λ  (steady state)
τ  = 1 / λ  (time to ~63% of the approach to N∞)

Here G is the total crew bioaerosol generation rate (particles/min, scales with crew size), Q is the volumetric airflow through the filter (m³/min, set by the air-exchange rate ACH and cabin volume V), and η is the single-pass HEPA capture efficiency. Because clearance is proportional to N itself, the system always relaxes exponentially toward N∞ = G/λ — this is the same first-order model used to size real spacecraft and cleanroom ventilation (ISS uses ~99.97%-efficient HEPA stages sized to the target ACH).

Crew size — more people means a proportionally higher generation rate G.
Air exchange rate (ACH) — how many cabin volumes are pushed through the filter per hour; raising it increases Q and lowers both N∞ and τ.
HEPA efficiency η — the fraction of particles captured per pass; a clogged or bypassed filter drops this and lets N∞ climb sharply since it appears in the denominator.
Cabin volume — a bigger volume dilutes the same generation rate but also needs more airflow to hit the same ACH.

The 3D view is time-accelerated (~4 simulated minutes per real second) so the exponential approach to N∞ is visible in seconds; particle density in the loop scales with the live concentration N(t), and particles crossing the glowing filter plane are captured with probability η per pass — the rest recirculate back into the cabin.

Bioaerosol particle HEPA filter plane

⚠ Couldn't load the 3D engineThree.js failed to load from the CDN. Check your connection and reload.