Their research turns building vents into an unlikely companion to food production. Many people will recognize these silver, mushroom-shaped structures that are visible on urban roofs: they form part of HVAC systems that pump out stale air and ventilate buildings with fresh air from outside. In buildings where lots of people live and work, the CO2 emissions from human respiration can reach surprisingly high levels, which is why these systems are needed to keep the air fresh and unpolluted.