Finer particulate matter (PM1) and risk of cardiovascular disease

By Dr. Liji Thomas, MDJan 29 2020

A new study published in the journal Environmental Science & Technology Letters reports that particulate matter smaller than a single micron in diameter, called PM1, is strongly linked to the occurrence of cardiovascular disease (CVD). This is thought to be due to its higher surface area to volume ratio.

It is already known that air pollution with small particles is associated with health problems, such as CVD and asthma. Many studies have looked at these issues in relation to particles smaller than 2.5 micrometers in diameter, but the current study went one step further to assess how PM1 affected health. This is the first nationwide study to look at the risk of death and PM1.

Outdoor air quality measurement. pm2.5 (particulate matter) sensors detecting small dust in the atmosphere. Image Credit: Chim / Shutterstock

The study

The study was carried out in China from January 2014 to December 2017. The air was monitored for the presence of PM1 all over the country, in 65 cities. These cities were chosen because the death rate per day was at least 3, and the air pollutant level, weather recording, and death data were all available on a yearly (or more frequent) basis during the study period. They were subclassified as belonging to the geographical south or north.

The data was then analyzed to look for any link between the number of deaths not due to accident, and PM1 exposure.

The findings

The average PM1 level in these locations was 37 ± 32 micrograms/m3 (μg/m3).

The results showed that there was a strong and proportional association between the PM1 level and the risk of non-accidental death. That is, with each 10 μg/m3 increase in PM1 the total deaths went up by 0.19%. This effect was 250% higher in the southern cities compared to that in the north, at 0.35% compared to 0.10%.

On the other hand, with each increase in PM1 levels by 10 μg/m3, the CVD risk went up by 0.29%.  When separately observed, the risk of CVD was increased by 0.44% and 0.19% in the southern and northern cities, respectively. In contrast, the risk of chronic obstructive pulmonary disease, and of respiratory disease like asthma, was not significantly increased.

Seasonal variations are also seen with increased effect of PM1 during the cold season. One reason for this disparity could be the lower concentrations of PM1 in southern regions, “which modifies its stronger effects” according to the researchers, who say the same effect is observed in PM2.5 and PM 2.5-10. Another reason could be that more of the PM1 came from mobile emissions in this region, which affects the death rates more.

In other words, the increased risk of CVD is 21% higher than the risk due to PM1 alone, with this level of increase in PM1 levels. The effect on stroke is an increase by 0.33% for each 10 μg/m3 increase in PM1. This is 24% and 30% higher than the effect due to PM2.5 and PM10.

Inhaled PM

Inhaled PM is thought to reduce normal vascular function, to speed up arterial thickening, and to push up CVD rates. When it is PM1, this excess in risk increase is due to the increase in ease of deposition of the finer particles deep within the lungs and in the blood vessels, compared to the larger particles.  Moreover, it has more toxins, metals, and organic compounds which are linked to lung and gene injury, and epigenetic changes.

PM1 originates from the emissions produced by combustion and from aerosols, and the ratio of PM1 to PM2.5 is over 0.70, a high figure when compared to the average ratio from other cities worldwide. The very high rate of industrial processes in China probably contribute to this altered ratio and push up the already high risk of CVD due to PM1 in air.

Other studies have suggested that PM can pass the blood brain barrier to act directly on the central nervous system. Both short-term and long-term exposure to PM1 has been associated with increased risk of death due to stroke, especially ischemic strokes. One systematic review reports a cumulative hazard ratio of 1.11 for stroke death with each 5 μg/m3 increase in PM2.5.

More research is essential to confirm the composition of PM1, to understand the true impact of PM on human health. However, the current findings indicate the immediate need to improve air quality in China, and to set up guidelines for PM1 control.