Around this season, people becomeoften feel uncomfortable because cedar pollen starts to spread. Around this season, people often feel uncomfortable because cedar pollen starts to spread.

"people become uncomfortable" is also correct, but not "become feel" at the same time.

Cedar trees have been planted for a long time to use as lumber, but imported lumber is more reasonablecommon these days. Cedar trees have been planted for a long time to use as lumber, but imported lumber is more common these days.

"Reasonable" is a weird word here. It means that something is logical, or sensible. But why is it more logical is not explained, so it sounds weird.

Therefore, a lot of cedar trees remain and spread tons of pollen every year. Therefore, a lot of cedar trees remain and spread tons of pollen every year.

Many Japanese people used a face mask before the epidemic of COVID-19COVID-19 pandemic, and pollen is one of the reasons why. Many Japanese people used a face mask before the COVID-19 pandemic, and pollen is one of the reasons why.

Epidemic: Outbreak of disease within one community or area. At most covers one country. Pandemic: Outbreak of disease across multiple countries or continents. So pandemic is the better word to describe COVID19.