Carbon Monoxide (CO), not to be confused with Carbon Dioxide (CO2), is an invisible, odourless and tasteless gas that is toxic to humans and animals. It is often referred to as the ?Silent Killer? because none of our senses can alert us to its presence; we can?t smell it, taste it or see it. The initial symptoms, tiredness and headaches, are similar to those of influenza, which can make it difficult to diagnose. High levels of Carbon Monoxide in the home can quickly render the occupants unconscious, leading to fatality within hours. In the UK alone, thousands of people are treated in hospital for CO poisoning every year. The longer CO gas is breathed in and the higher its concentration, the worse the symptoms become, including loss of balance, loss of vision and memory and eventually unconsciousness. This can happen within just a few minutes or hours, depending on the amount of CO in the air. CO alarms give a full alarm, before the first symptoms occur: The higher the CO concentration, the quicker the device will give an alarm.