PM2.5 particles, those measuring  less than 2.5 microns, easily pass through our lungs and into our bloodstream and are then deposited throughout the organs of our bodies. 

In 2021, the WHO decreased the safe annual average exposure for PM2.5 to an AQI of 28.  These changes reflected new evidence showing negative health effects as a result of low levels of exposure. WHO decreased the 24-hour average recommended exposure level to less than AQI of 62. This 24 hour level should only occur 3-4 times per year to avoid health risks.

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Source: https://iris.who.int/bitstream/handle/10665/345329/9789240034228-eng.pdf?sequence=1

"Both short-term acute exposure to high levels and long-term chronic exposure to low levels of particle pollution can cause serious harm. Short-term (hours to days) acute exposures to fine particles can trigger cardiovascular events, hospitalization episodes, and mortality. Long-term (months to years) chronic exposures to fine particles can increase the risk of strokes, coronary heart disease and cause premature deaths."

Source: https://www.lung.org/clean-air/outdoors/who-is-at-risk