In the current heavy-duty engine and light-duty diesel vehicle exhaust emission legislation Particle Number (PN) limits for solid particles >23 nm are prescribed. The legislation was extended to include Gasoline Direct Injection (G-DI) vehicles since September 2014 and will be applied to Non-Road Mobile Machinery engines in the future. However there are concerns transferring the same methodology to other engine technologies, where higher concentration of sub-23 nm particles might exist. This paper focuses on the capabilities of existing PN measurement equipment on measuring solid particles smaller than 23 nm. More specifically, it is investigated: 1) whether it is feasible to easily modify existing systems to measure lower particle sizes, 2) whether all volatile particles can be removed efficiently in the PN measurement systems, 3) whether any artifacts happen in the PN systems (e.g. formation of non-volatile particles due to pyrolysis), and 4) whether by lowering the lower size the measurement uncertainty increases significantly. The main conclusions are: 1) It is relatively easy to modify the systems to measure from 10 nm. 2) The volatiles are not always removed efficiently in the PN measurement systems. The major issue is re-nucleation of sulfuric acid downstream of the evaporation tube. However, these particles typically do not grow to 23 nm or to 10 nm with high dilution. 3) There are indications of formation of 10 nm solid particles from hydrocarbons and sulfuric acid in the PN systems. 4) With lower size of the PN equipment, the measurement uncertainty will increase due to differences in the losses between the PN systems. However the increase of uncertainty is estimated to be <5% for measurements >10 nm, when no separate solid sub-23 nm core mode exists. Recommendations for proper measurements below 23 nm are given. The main conclusion of this study is that, in case that it is proven that there is a need for solid sub-23 nm particle measurements, the existing PN systems could be relatively easily adjusted to measure from 10 nm without increasing significantly the measurement uncertainty. Sub-10 nm measurements need major changes of the PN systems.