Pocket sized instrument for measuring toxic gases and oxygen deficiency or enrichment in ambient air in the work place. Depending on which sensors are fitted, it can reliably warn of the presence of harmful concentrations of carbon monoxide, hydrogen sulphide, oxygen, ammonia, hydrocyanic acid, chlorine, carbon dioxide, hydrogen phosphide, sulphur dioxide, nitrogen dioxide or organic vapours. Robust impact-proof housing that is also resistant to corrosive chemicals and light weight body with 2 alarm thresholds.