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Cortical Excitations, Cortisol Excretion and Estimation of Tolerable Nightly Over-Flights.

Noise induces cortisol excretion even below the awakening threshold. This is based upon the existence of very close subcortical central nervous connections between parts of the auditory system (e. g. amygdala) showing typical plasticity effects and the hypothalmic-pituitary-adrenal (HPA)-axis. Repeated noise events (e.g. over-flights during nigh-time) will lead to accumulation of the cortisol concentration in blood. This happens because its time constant of exponential decrease is about 50 to 10 times larger than that one for adrenaline and noradrenaline. A twofold attempt has been made to calculate the cortisol accumulation using an initial value of noise induced small cortisol increase (rounded value 14 ng/ml) at the nightly threshold of beginning vegetative overreaction around 53 dB(A). A mean time-constant of 64 min has been applied based upon experimental studies. Using in a first step the range of minimal and maximal normal cortisol values as border line and taking into account a relation between peak sound pressure level and cortical excitation given by a power function (exponent 0.32, based on evoked potential studies in man) results in a formula to estimate tolerable events during night-time periods (over-flights in a given time range). Examples of results for 8 hours in the night are for instance values of 11 events with 55 dB(A) indoor peak level or 5 events with 75 dB(A) indoor peak level respectively. Those values of tolerable nightly noise events estimated on the basis of physiological processes and peak levels cannot be recalculated as or compared with equivalent sound levels.