Using & analysing international reporting formats exclusively does not provide a true picture of whether a system is in control or out of control.
Although all of the international formats are based on a sound scaling method, they are all sensitive to a change of just one count at any concentration. For example, ISO 14 denotes that you have between 80 & 160 particles of a given size in your system. If the concentration in the system changes to 161, the APC will output a result of ISO 15. Conversely, if the count drops to 79, then the result will be ISO 13.
The question is, does a change of one particle count justify a decision to take action or not? What must be considered is at what point does cumulative effect make a difference to the function of the system?
Although it is easy to arbitrarily set limits, we need to understand how close we are to exceeding them. If ISO 14 is your upper limit of contamination, and at no costs should the system exceed ISO 14, then it would not be very responsible to be unknowingly operating at 99% or even 80% of your upper control limit (Certainly not for an extended period of time).
Although international reporting formats are useful, and in a lot of cases are practically suitable, it is always good to understand the importance of detailed counts to paint a clearer picture of the situation and to set achievable control limits.