The first issue was ammunition. Before the war it was recognised that ammunition needed to explode in the air. Both high explosive (HE) and shrapnel were used, mostly the former. Airburst fuses were either igniferious (based on a burning fuse) or mechanical (clockwork). Igniferious fuses were not well suited for anti-aircraft use. The fuse length was determined by time of flight, but the burning rate of the gunpowder was affected by altitude. The British pom-poms had only contact-fused ammunition. Zeppelins, being hydrogen filled balloons, were targets for incendiary shells and the British introduced these with airburst fuses, both shrapnel type-forward projection of incendiary 'pot' and base ejection of an incendiary stream. The British also fitted tracers to their shells for use at night. Smoke shells were also available for some AA guns, these bursts were used as targets during training.