According to ECG Environmental Brief Article, atmospheric particulate matter (PM)  is a central component of the atmospheric chemical and climate system, a major air pollutant harmful to human health, and a component of biological systems and global biogeochemical cycles. [1] To keep it short and simple, PM is how bad is the air around you. High PM means that the surrounding air contain a lot of particles. If the air around you looks cloudy, the PM is high. Otherwise, the PM is low and you are safe-to-go. 

With a high PM of 2.5[2], you would be seeing everything cloudy (haze). This makes you having difficulty to see from far because the particles in the air block your sight. 

For futher enquires, please do visit https://www.epa.gov/visibility

In a high PM air, the particles in the air will stain at water, soil and buildings. These tiny particles will change the pH value of water, causing acid rain and also making water dangerous to animals. The rain turns into acid rain and further harm buildings, Slowly but surely, the buildings will collapse, maybe in another few tenth years. Changing the attribute of soil will also make plants unable to live healthily or even wilt.

Feeling bad all day long? Maybe because of the air around you! The particles in the air might trap inside your lungs as you breath, or even be in your bloodstream. This is a serious problem as this can cause you breathing problems. Harden your blood vessels, darken your lungs, you name it. 

Knowing that PM level is so important to your health, how can you know whether you are safe? You can always do a quick check by following the experiment in https://www.education.com/science-fair/article/counting-air-particulate-matter/. 

According to Official Website of Malaysian Meteorlogical Department, the PM in my area is as below:

Petaling Jaya

The monthly mean PM-10 concentration recorded is 27.8 µg/m3 with the highest value of PM-10 concentration, 58.0 µg/m3 and the lowest value, 11.5 µg/m3. Each were recorded on the 12th and 22nd of January respectively. The PM-10 monthly mean concentration for January 2017 show a decreased by 21.5% compared to the long term mean concentration of 35.4 µg/m3. The PM-10 concentration shows an increasing trend from 8th to 12th of the month due to no rainfall recorded for 9 consecutive days at the station since 4th until 12th January.

[3]

Reference:

1.http://www.rsc.org/images/environmental-brief-no-4-2014_tcm18-237725.pdf

2.https://www.epa.gov/pm-pollution/applying-or-implementing-particulate-matter-pm-standards

3. http://www.met.gov.my/en/web/metmalaysia/publications/bulletinpreview/monthlyairquality