 The distance between probes varies but rarely exceeds 300 metres.The typical regulatory limit of methane here is 50,000 parts per million (ppm) by volume, or 1% methane and 1.5% carbon dioxide above geological background levels in the UK (see " Guidance on the monitoring of Landfill Gas " LFTGN03, EA 2004).Ambient air samplers are used to monitor the air around a landfill for excessive amounts of methane and other gases.