Updated 28 December 2023
Light & seasonality: How much UV are you actually receiving?
Low temperatures
Absolute humidity (aerosoltransmission)
Lifestyle factors, such as crowding together indoors
Seasonality of ultraviolet (UV) radiation
Nearly all human diseases related to respiratory pathogens show seasonality. This paper reviews the studies on how environmental factors affect flu rates worldwide.
Flu data shows that the numbers vary predictably with seasons, whether it is annual flu numbers or a pandemic.
These are studies from varied locations: the cold Nordic countries such as Sweden and Norway, the states across the USA, the subtropics such as Taiwan and Okinawa in Japan, to tropical islands such as Singapore.
These factors are seasonality of low temperatures, absolute humidity (aerosoltransmission), or of dry air, crowding together indoors during the winter, travel patterns, vacations, seasonality of ultraviolet (UV)radiation from the sun that might kill pathogens, circadian rhythms of hormones.
Read the research study here: The seasonality of pandemic and non-pandemic influenzas: the roles of solarradiation and vitamin https://doi.org/10.1016/j.ijid.2010.09.002
How much UV are you actually receiving?
The amount of UV received depends on the zenith angle (variable with season, latitude, and time of day),cloud and snow cover, aerosols, and the thickness of the ozone layer. Researchers use a radiative transfer model to calculate global solar UV irradiances.
In their study on the Nordic countries, the vitamin D level is maximal about a month after the time of maximal rate of synthesis, which occurs close to mid summer. This is due to the fact that the vitamin D level here is determined as the concentration of 25-hydroxyvitamin D in serum, and that the formation of this metabolite from pre-vitaminD, via vitamin D (mainly in the liver), takes around one week.
The greater the UV radiation, the lower the virality
The lowest pneumonia and influenza mortality rates were seen in the areas with the highest solar UVB irradiance and lowest latitudes (these being good indicators for high levels of vitamin D).
In Singapore, the variations in vitamin D photosynthesis decrease as the equator is approached. In fact, as the curve for Singapore(18N) shows (Figure 5), there are two minor maxima per year,located almost symmetrically around the midsummer minimum. The reason why it is not totally symmetrical is due to asymmetrical ozone layer and the variability in cloud cover.
In Singapore, while there is no real flu seasonality, the influenza waves start during periods of low vitamin D photosynthesis. These peaks maybe related to humidity, or possibly to contamination from seasonal influenzas in the southern and northern hemisphere, and to the seasonal variation in vitamin D photosynthesis.
UV radiation affects the immune system
Children who were regularly exposed to artificial UVB radiation had around two times lower incidence rates of upper respiratory tract infections, influenza, and sore throat than non-exposed children, and the phagocytic activity of macrophages increased.
Flu mortality rates tend to be lower in rural areas compared to urban areas and is closely related to vitamin D status. People living in rural areas have significantly higher vitamin D levels compared to those living in urban areas.
