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Don't know application of this circuit, but it can detect pulses <300nA all circuits with this sensitivity are sensitive to external noise.

Someone can make 300nA circuit with <1Mohm resistors. Sensitivity to leakage and mains hum will be similar.

It is good practice to keep it clean. Did anyone measured this dramatic change? I can touch 100Mohm (1206) and there is no dramatic change. 

It depends on voltage. Guard rings on PCB can nullify voltage and surface leakage.