1. A nitrous oxide detector (400), comprising a gas sensor (100) having: a measurement chamber (102) for receiving ambient gas to be measured; an optical source (103) for emitting radiation in the measurement chamber (102); a radiation detector (104) sensitive to radiation emitted by the optical source (103), the radiation being configured to be detected by the radiation detector (104) following passage through the measurement chamber (102); and the radiation detector (104) being configured to measure occurrence of nitrous oxide (N2O) in the gas being measured; the gas sensor being characterised in: a diffusion layer (131), the diffusion layer (131) being configured to allow diffusion of ambient gas into and out from the measurement chamber (102), and the diffusion layer (131) comprising means (133) for preventing diffusion of carbon dioxide (CO2) in the ambient gas into the measurement chamber (102), the nitrous oxide detector (400) further comprising: processing means (402) for receiving measurements of the occurrence of nitrous oxide (N2O) from the gas sensor (100); and the processing means (402) being configured to generate a signal when the measurements of the occurrence of nitrous oxide (N2O) exceed a predetermined threshold.

2. A nitrous oxide detector (400) according to claim 1 , wherein: the diffusion layer (131) comprising absorbing means, such as one or more absorbents, for absorbing carbon dioxide (CO2) in the ambient gas passing through the diffusion layer (131) to thereby prevent carbon dioxide (CO2) in the ambient gas from entering the measurement chamber (102).

3. A nitrous oxide detector (400) according to claim 1 or 2, wherein: the diffusion layer (131) is designed to allow a diffusion rate of nitrous oxide (N2O) being such that the concentration of nitrous oxide (N2O) in the measurement chamber (102) reflects a change in concentration of nitrous oxide (N2O) in the ambient gas within a first predetermined period of time.

4. A nitrous oxide detector (400) according to claim 3, wherein: the diffusion layer (131) is designed such that a diffusion rate through the diffusion layer is dependent on the thickness of the diffusion layer (131).

5. A nitrous oxide detector (400) according to any one of the claims 1 -4, wherein: the diffusion layer (131 ) comprises a filter.

6. A nitrous oxide detector (400) according to claim 5, wherein: the filter comprises sodium hydroxide and/or calcium hydroxide as absorbents for absorbing carbon dioxide (CO2) being present in the gas entering the measurement chamber (102).

7. A nitrous oxide detector (400) according to any one of the claims 1 -6, further comprising: receiving means for releasably receiving corresponding means of the diffusion layer, or vice versa, for releasable attachment of the diffusion layer (131 ) to the sensor body so as to allow replacement of the diffusion layer (131).

8. A nitrous oxide detector (400) according to any one of the claims 1 -7, further comprising: the processing means (402) being configured to accumulate a presence of nitrous oxide (N2O) over time, and the processing means being configured to generate the signal when an accumulated occurrence of nitrous oxide (N2O) exceeds the predetermined threshold.

9. A nitrous oxide detector (400) according to any one of the claims 1 -8, the diffusion layer (131) comprising at least one absorbent being consumed by absorbing carbon dioxide (CO2) and requiring regular replacement, the nitrous oxide (N2O) detector further comprising: means for determining a time of operation following a replacement of the diffusion layer (131), and means for generating a signal representing a requirement for replacing the diffusion layer (131) following lapse of a second predetermined time.

10. A nitrous oxide detector (400) according to any one of the claims 1-9, characterised in that it is designed to be carried by medical staff throughout the workday while carrying out regular work.

11. A nitrous oxide detector (400) according to any one of the claims 1 -10, characterised in that the measurement chamber (102) is designed such that the optical pathway between the optical source (103) and the radiation detector (104) exceed the possible optical pathway between the optical source (103) and the radiation detector (104) when arranged within direct line of sight of each other within the measurement chamber so as to allow a reduction in size of the nitrous oxide detector while still providing a minimum desired optical pathway in the measurement chamber (102).

12. A method for determining an occurrence of nitrous oxide (N2O) using a nitrous oxide detector comprising a gas sensor (100), the gas sensor comprising: a measurement chamber (102) for receiving ambient gas to be measured; an optical source (103) for emitting radiation in the measurement chamber (102); a detector sensitive to radiation emitted by the optical source (103), the radiation being detected following passage through the measurement chamber (102), and the radiation detector (104) being configured to measure occurrence of nitrous oxide (N2O) in the gas being measured; a diffusion layer (131), the diffusion layer (131) being configured to allow diffusion of nitrous oxide (N2O) into and out from the measurement chamber (102), and the diffusion layer (131) comprising means for preventing diffusion of carbon dioxide (CO2) in the ambient gas into the measurement chamber (102), the method comprising the step of: receiving measurements of the occurrence of nitrous oxide (N2O) from the gas sensor (100); and generating a signal when the measurements of the occurrence of nitrous oxide (N2O) exceed a predetermined threshold.

13. Computer program comprising instructions which, when the program is executed by a computer, cause the computer to carry out the method according to claim 12. 14. Computer-readable medium comprising instructions which, when executed by a computer, cause the computer to carry out the method according to claim 12.