<Tr> <Td> n - Butane </Td> <Td> 12 </Td> <Td> 14.5 </Td> </Tr> <Tr> <Td> Isobutane </Td> <Td> 12 </Td> <Td> 15 </Td> </Tr> <P> Limiting oxygen concentration for solid materials </P> <Table> <Tr> <Th> Material </Th> <Th> Nitrogen / Air </Th> </Tr> <Tr> <Td> PE - HD </Td> <Td> 16.0 </Td> </Tr> <Tr> <Td> PP </Td> <Td> 16.0 </Td> </Tr> <Tr> <Td> PMMA </Td> <Td> 15.9 </Td> </Tr> <Tr> <Td> PVC </Td> <Td> 16.9 </Td> </Tr> <Tr> <Td> PE - LD </Td> <Td> 15.9 </Td> </Tr> <Tr> <Td> Fir wood </Td> <Td> 17.0 </Td> </Tr> <Tr> <Td> Corrugated board </Td> <Td> 15.0 </Td> </Tr> <Tr> <Td> Cardboard palletised </Td> <Td> 15.0 </Td> </Tr> <Tr> <Td> Paper </Td> <Td> 14.1 </Td> </Tr> </Table>

What is the minimum level of oxygen required to sustain a fire