Texas freeze led to release of tons of air pollutants as refineries shut

• Shutdowns led to the refineries flaring, or burning and releasing gases
• What caused huge Texas blackouts - a visual explainer

The largest oil refineries released tons of air pollutants into the skies over Texas this week, according to figures provided to the state, as one environmental crisis triggered another.

Related: Texans rally to help neighbors amid big freeze as officials are caught cold