anoxic

Anoxic describes a condition in which oxygen is absent or extremely limited. The term comes from the Greek an-, meaning without, and oxys, meaning oxygen, and it is used to describe environments or situations where dissolved oxygen is effectively unavailable.

In nature, anoxic conditions are common in waterlogged soils and in the deeper layers of stratified lakes and oceans. In such settings, microbial respiration can proceed with electron acceptors other than oxygen, such as nitrate or sulfate, and reduced compounds like hydrogen sulfide can accumulate. Anoxia also characterizes sediments that are starved of oxygen, which can slow decomposition and lead to the accumulation of black, sulfur-rich layers.

Dissolved oxygen thresholds are used to judge oxygen status. Anoxic generally refers to almost no measurable oxygen, often defined as dissolved oxygen near 0 mg/L, while hypoxic describes low but detectable oxygen, typically around 0.5–2 mg/L. The exact thresholds vary by context and organization.

Impacts of anoxic conditions include disrupted aquatic ecosystems, the formation of dead zones, and altered nutrient cycling. Anoxia is exploited in wastewater treatment for denitrification and in some food packaging to prevent oxidation by creating an oxygen-free atmosphere. In medicine, anoxia refers to a complete lack of oxygen supply to tissues and can occur during events such as cardiac arrest or severe respiratory failure.