Have you ever looked outside and noticed that the sky suddenly appears gray, smoky, or less clear than usual? It may be tempting to think that it is simply cloudy weather, but haze can sometimes indicate that tiny particles and other pollutants are suspended in the air.

One of the pollutants commonly associated with hazy conditions is PM2.5, or fine particulate matter measuring 2.5 micrometers or smaller in diameter. These particles are small enough to travel deep into the respiratory system. According to the World Health Organization (WHO), PM2.5 can penetrate deep into the lungs and may enter the bloodstream, which is why exposure is associated with respiratory and cardiovascular health risks.

PM2.5 can come from different sources. Vehicle emissions, industrial activities, waste burning, power generation, and fires can all contribute to particulate pollution. Haze can also travel far from where it originated. This is known as transboundary haze, which occurs when smoke and fine particles are carried across national or regional boundaries by prevailing winds.

This was recently observed in Northern Mindanao. The Environmental Management Bureau Region X reported that haze in some parts of the Philippines was associated with smoke from forest and land fires in Kalimantan, Indonesia. The bureau noted that prevailing winds could carry smoke and fine particulate matter across national borders. Monitoring stations in Northern Mindanao recorded varying air-quality conditions, including elevated readings in Iligan City.

By September 21, local government units in Northern Mindanao were again warning residents about outdoor exposure as haze affected air quality. The Environmental Management Bureau-Northern Mindanao reported PM2.5 Air Quality Index readings ranging from “unhealthy for sensitive groups” to “acutely unhealthy” at its monitoring stations earlier in the day.

So, what should people do when air quality becomes poor?

The first step is to check official air-quality updates instead of relying only on how the sky looks. Conditions can change throughout the day, and a hazy-looking sky does not always tell the full story about the concentration of pollutants in the air.

People can also reduce prolonged outdoor exposure when air quality is poor. Those who are particularly vulnerable, including children, older adults, pregnant individuals, and people with existing health conditions, may need to take additional precautions. The WHO identifies these groups among those who can face greater risks from air pollution exposure.

Understanding PM2.5 also helps explain why air quality deserves attention even when pollution cannot be seen. Haze may disappear as weather conditions change, but the need to monitor and reduce exposure to air pollution remains.

The next time the sky looks unusually hazy, it may be more than a change in scenery. It can be a reminder to check the air we cannot always see.