In general, outdoor levels of CO2 are much much lower than indoor levels. That means, opening windows will typically release CO2 from your room to the outside. Outdoor CO2 levels are typically in the range of 300-600ppm. However, under extreme circumstances, for example, next to a very busy road, they may reach up to 1000ppm.
By contrast, CO2 levels in a room can often reach 1000-2000ppm if there are multiple people in the room.
Learn more: how does keeping windows shut affect carbon dioxide and oxygen levels »
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