A vacuum pumping system which comprises a plurality of vacuum pumping arrangements for evacuating a flammable gas stream and exhausting the gas stream through an exhaust outlet. A housing houses the vacuum pumping arrangements and forms an air flow duct for an air flow for mixing with the exhaust gas stream output from the exhaust outlet in a mixing region in the housing. An airflow generator generates an air flow through the air flow duct to cause mixing of air with the flammable gas stream to a percentage of the flammable gas in the air flow lower than the lower flammability limit of the flammable gas. An airflow sensor senses the flow of air for determining if the air flow is sufficient to dilute the flammable gas to lower than said percentage.