The transcanal LLLT was found to recover noise induced hearing loss (NIHL) but the LLLT was performed
immediately after the induction of NIHL. The aim of this study was to find an optimal window time to treat and
recover a NIHL with LLLT. Bilateral ears of 6SD rats (12ears) were exposed to noise. Left ears of the rats were
irradiated with a LLLT (830 nm, 594 J/cm2 per day) for 12 days, starting 3 days and 7 days post exposure to noise.
Right ears were used as control ears. The hearing levels were measured at each frequency of 4, 8, 12, 16, and 32 kHz
before and after the noise exposure and post 12th irradiations. The initial hearing levels in all frequencies before and
after the noise exposure were 26.5, 24.5, 24.0, 24.0 and 24.5 dB SPL and 63.5, 64, 71.5, 73.5 and 67.5 dB SPL in 4,
8, 12, 16 and 32 kHz, respectively in 6 ears. After 12th irradiation, the thresholds of the LLLT treated left ears of the
3-day group recovered significantly compared to those of the untreated right. However, for the 7 day group, the
recovery of the LLLT treated left ears was not significantly improved compared to that of the untreated right. The
results of this study suggest that the optimal window time to treat NIHL with LLLT was within 3 days from the
exposure to noise but the hearing failed to recover if the LLLT was started 7 days post exposure to noise.