<Tr> <Td> Carbon monoxide </Td> <Td> 0.040 </Td> <Td> 0.033 </Td> <Td> 0.208 </Td> </Tr> <Tr> <Td> Sulfur dioxide </Td> <Td> 0.001 </Td> <Td> 1.122 </Td> <Td> 2.591 </Td> </Tr> <Tr> <Td> Nitrogen oxides </Td> <Td> 0.092 </Td> <Td> 0.448 </Td> <Td> 0.457 </Td> </Tr> <Tr> <Td> Particulates </Td> <Td> 0.007 </Td> <Td> 0.084 </Td> <Td> 2.744 </Td> </Tr>

When did they add smell to natural gas