Japan's Ministry of Environment said smog blowing from Beijing should begin to improve by Wednesday, after it advised those in the west of the country to wear masks and close their windows.
But Hongkongers have so far not canceled trips to Japan as a result of air pollution issues, while tours to Beijing are down 20percent from last Lunar New Year, tour agencies said.
Three cities in Hiroshima prefecture recorded 40 micrograms per cubic meter of PM2.5 air particles on Thursday, exceeding the 25 micrograms set by the World Health Organization.
The small particles may penetrate the lungs and cause respiratory illness, experts say.
For the past few weeks, Beijing has been smothered with smog, with the PM2.5 level at 500, well above the "hazardous" level of 300.
The US National Aeronautics and Space Administration photographed visible smog from space blowing to western Japan last week.
The air is expected to improve after Wednesday, the Japanese environment ministry said.
However, the smog has not affected more than 200 Hong Kong tours booked for the Lunar New Year period.
EGL Tours executive director Steve Huen Kwok-chuen said customers are yet to express their concern to him over the air quality, and he believes that a few days in Japan will not affect travelers' health.
"The buildings in Japan are relatively sparse and lower and there are large open spaces. I believe the air pollutants will not affect travelers as much as those living there," Huen said.
But he said he has advised Beijing tour group members to wear masks, adding that tours to the capital are down 20percent from the same period last year.
Kenneth Tsang Wah-tak, a specialist in respiratory medicine, said masks cannot protect wearers from PM2.5 particles as they can infiltrate the smallest of gaps.
"There is 10 to 20percent higher chance of triggering chronic lungs and heart diseases by breathing in polluted air," Tsang said.
He said those with asthma should take inhalers to Japan as poor air quality may trigger an attack.