It is often numerically convenient to reduce models in two-dimensions to one-dimension. This can be done formally through the use of centre manifold techniques, or informally using physical reasoning. We investigate the extent to which flammability limits in a two-dimensional slab are accurately represented by the values in the corresponding one-dimensional slab. We use a simple chemical mechanism containing exothermic and endothermic reactions that has been used to model the combustion of hydrocarbon fragments produced by polymer pyrolysis.