ISS Life Support Cooperation Network

Crew & Hardware

Total crew aboard 4

IMV fan flow rate 4.0 m³/min

Cabin CO₂ (ppCO2, mmHg)

US Lab (Destiny)NASA, has CDRA0.30

Node hubshared, no scrubber0.30

Russian segment (Zvezda)Roscosmos, has Vozdukh0.30

SMAC alarm limit5.30 mmHg

How it works

Each module is a well-mixed compartment. Crew respiration adds CO₂, an onboard scrubber removes it, and Inter-Module Ventilation (IMV) fans exchange air with the neighbouring compartments — the literal hardware that makes the partner segments need each other:

dP_i/dt = K·crew_i / V_i          (crew respiration)
        − k_s·(P_i − P_base)·[scrubber_i on]   (CDRA / Vozdukh)
        + Σ_j (Q_fan / V_i)·(P_j − P_i)        (IMV exchange with neighbour j)

P_i — CO₂ partial pressure in module i (mmHg); V_i — module free volume (US Lab 106 m³, Node 60 m³, Zvezda 90 m³).
Scrubber term — CDRA (US Lab) and Vozdukh (Zvezda) each pull their own module's CO₂ toward the 0.30 mmHg cabin baseline. The Node has no scrubber of its own — it only breathes through the ducts.
Exchange term — the IMV fan flow rate sets how fast the ducts equalise concentration between adjacent modules. Turn either scrubber off: raise the fan flow and the healthy segment's hardware pulls the whole station down; leave it low and the failing segment's CO₂ climbs on its own toward the 5.30 mmHg alarm limit.

This is the real engineering reason ISS partner segments share one atmosphere loop instead of running independently: cross-module ventilation gives every agency's life-support hardware redundancy for its neighbours, at the cost of also sharing a fault if nobody reacts.

Low CO₂ (near baseline) Elevated Near/above alarm limit

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