In generate a great many worlds with exotic atmospheres. Corrosive atmospheres, per First In , are usually high in ammonia. With most Traveller materials, this will not pose a problem. Insidious atmospheres such as chlorine or fluorine, popular in science fiction, are probably impossible, but these chemicals could occur as pollutants in habitable or corrosive atmospheres. Atmosphere types D-F are breathable atmospheres with exotic shape.
Type D is an atmosphere too dense to breathe, but habitable at high altitude.
Type E designates an elliptical world, with oceans and high pressure at the poles and high sierra at the equator. It might be a terraformed Desert world, but should have size 4 or 5. This may be amended to size 6, or blamed on the Ancients. Ordinarily, a habitable world of size 4 might be a terraformed Desert world, but Daryen's extreme antiquity makes this unlikely. First In does provide a somewhat more realistic method of correlating worlds with population and tech levels.