During the latest haze episode, the air is filled with both coarse and fine particles released by large forest and land fires, explained Adjunct Associate Professor Adrian Kee, head and senior consultant at National University Hospital’s Division of Respiratory & Critical Care Medicine.

Kee noted that while coarse particles settle within a day, fine particles stay aloft for weeks and can travel hundreds of kilometres. The haze therefore contains particulate matter smaller than 10 micrometres, including fine particles under 2.5 micrometres.

These sub‑2.5‑micrometre particles are the ones that trigger coughing, make breathing difficult and aggravate asthma, according to Kee. They can also irritate the eyes, throat and nose.

An N95 respirator can capture particles as small as 0.1‑0.3 micrometres. Kee said studies show such masks are at least 95 % effective when they fit correctly and are used as intended.

Dr Esther Pang Pee Hwee, senior consultant at Tan Tock Seng Hospital’s Department of Respiratory and Critical Care Medicine, stressed that the mask must be the right size, undamaged, clean and tightly sealed to work properly.

When the 24‑hour PSI reads 101‑200 (unhealthy), Kee advises people with chronic lung or heart disease, older adults, pregnant women and children to avoid prolonged or strenuous outdoor activity. If the PSI rises to 201‑300 (very unhealthy), the same groups should stay indoors as much as possible. Above 300 (hazardous), Pang recommends wearing a properly fitted N95 and remaining indoors.

Surgical and fabric masks do not filter particles of 2.5 micrometres or smaller. Kee explained that these masks only catch larger irritants and usually have a looser fit, allowing fine haze particles to slip around the edges.

Before using an N95, check the expiration date, look for tears or deformed shape, and test the seal by feeling for air leaks while breathing. Pang suggests consulting a health professional if you are unsure about the mask’s condition or your need for one.

In short, the fine haze particles that cause coughing and asthma flare‑ups can only be blocked effectively by a well‑fitted, undamaged N95 mask. Surgical or cloth masks give limited protection, so vulnerable groups should limit outdoor exposure and follow the PSI‑based guidance.