Complex sentence: Emissions from electronic cigarettes are not comparable to environmental pollution or cigarette smoke as their nature and chemical composition are completely different, although particle size distribution is similar with the majority of particles in the ultrafine range (modes, ≈ 100--200).
Split into simpler sentences: Emissions from electronic cigarettes are not comparable to environmental pollution or cigarette smoke. The nature and chemical composition of the components of e-cigarette vapor is completely different from those in cigarette smoke although particle size distribution is similar, with the majority of particles in the ultrafine range (modes, ≈ 100--200).