Washington : The US Army is working on next generation eco-friendly smoke grenades to provide cover for its troops in the battle field. The new grenades are being tested as a replacement for current smoke grenades, based on Second World War technology, that are used by US troops to mask themselves from enemy fire. The US Army Edgewood Chemical Biological Centre (ECBC) is working on a new formula to replace the 70-year-old HC Screening Smoke Grenade. Four basic smoke compositions are being considered. First is a zinc-free composition, HX, similar to the hexachloroethane composition, but one that does not produce a toxic, irritant smoke containing zinc chloride. The metals other than zinc produce smoke as dense as the HC, but without high amounts of acidity.