Density: About 1.5 times heavier than air (CO2 density relative to air)

Carbon dioxide has a density of approximately 1.98 kg/m³ at standard temperature and pressure, compared with 1.29 kg/m³ for atmospheric air — making it roughly 1.5 times heavier than air. This density difference has practical safety implications across every Indian CBG plant, where CO₂ is the second-largest constituent of raw biogas (typically 30–45% by volume) and is the principal off-gas stream from amine, PSA, and water scrubbing upgrading systems.

Because CO₂ sinks, it accumulates in low-lying enclosed spaces — pits, trenches, sumps, digester inspection chambers, and basement-level equipment rooms — even when the bulk atmosphere outside appears normal. Concentrations of 4% cause headache and breathing difficulty, 8% causes unconsciousness within minutes, and 10% or higher is rapidly fatal through simple asphyxiation. Unlike H₂S, CO₂ has no odour, so workers receive no sensory warning before losing consciousness.

Standard safety provisions in Indian CBG operations include:

• No-entry pits — confined-space entry permits, forced ventilation, and gas testing before any worker descends into below-grade chambers
• CO₂ detectors — fixed sensors at floor level in pump houses and digester foundations, alarming at 0.5% (5,000 ppm) and 1.5%
• Vent stack height — CO₂ off-gas vented from upgrading skids must discharge above building rooflines, typically 3–5 m above grade, to allow atmospheric dispersion
• Bottom-of-tank ventilation — exhaust fans drawing from low points rather than relying on roof vents that miss the heavy gas

The density characteristic also influences design of food-grade CO₂ recovery units, where the same property that makes CO₂ a hazard in a pit makes it easy to liquefy and bottle — it settles into the bottom of capture vessels under modest compression, simplifying purification. The hazard and the commercial opportunity are two sides of the same physical property.