Why can't many old people do it every February or so?

According to ancient books and Taoism, it is the time of Yin Qi in the world around every year (so Tomb-Sweeping Day will be in those days after the year). The doctor's statement is that the air quality and weather are the reasons around every year, and the old people's physique is inherently bad. In winter, the water molecules in the air are too thin, which can easily cause other reasons, so the old people will die at this time of the year.