If the WHO guidelines recommend noise levels below 53 dB Lden, why did the study use 55 dB Lden as a threshold for harmful levels of noise?

Even though the World Health Organization (WHO) recommends reducing exposure to road traffic noise below 53 dB Lden, the majority of noise maps only include exposure levels starting at 55 dB Lden. To conduct a harmonized analysis for all cities the research team calculated population exposure to road traffic noise starting 55 dB Lden, in accordance with the available data.