Normal ambient CO2 level is about 400ppm and that according to a USDA study, an average person produces about 450 liters (900 grams) of CO2 per day.

That means that you can detect one or more persons in confined spaces even with 2 – 3,000 ppm of CO2. The more persons or longer stay in that space will produce more CO2 faster.

Sigma 2000 CO2 with an internal air sampling pump, and an external 300mm probe is a portable carbon dioxide detector that can be used for the detection of people hiding in confined spaces such as trailers, rooms and vehicles. It works primarily by detecting the elevated levels of carbon dioxide caused by human breathing.
Sigma 2000 CO2 uses an infra-red carbon dioxide sensor with a range of 0 to 5000 ppm .

Stowaway Detection Applications:
• Air Cargo Containers
• Container Terminals
• Transport Security
• Security Agencies
• Correctional Facilities
• Confined Spaces
• Law Enforcement
• Search and Rescue
• Border Posts