What is maximum linear reading in atomic absorption?

That seems an odd maximum for the range that the absorption can be considered to respond in a linear fashion.

You do understand that absorption of light varies with concentration, which is the reason we can use AA to measure unknown concentrations, I assume. The issue is that absorption does not respond in a linear fashion to changes in concentration. It is only approximately linear over a fairly narrow range (we use concentrations where the absorption curve is almost straight, rather than concentrations where the curve has a significant non-linear behavior).

So, the instrument can detect the target species at concentrations outside the "linear" range, but the relationship to real concentration is not linear, so the assumed value for the measured absorption will not be correct. Usually, higher concentrations will be falsely assumed to be lower than they truly are. The reverse is generally true on the low concentration end, but normally the detection equipment is not designed to measure the small changes that will happen at very low concentrations (gives a result called "non-detection" even if there is some there).

In other words, there is a window of concentrations that actually can be measured, where the measured absorbance is directly (linearly) related to the concentration. It is not linear outside of that range. Although it depends on what is being measured, the ranges where there is linear response tend to be 1-10 ppm or 1-25 ppm, and that sort of thing, which is why I wonder about that odd 0.6 ppm value as a maximum.

Hi yes a gieger counter can read a lot more. however safe levels are a much different subject.