Closing windows can reduce exposure to outdoor haze but does not effectively prevent the infiltration of fine particulate matter known as PM2.5, according to public health experts. These microscopic particles, measuring 2.5 micrometres or smaller, are capable of entering homes through gaps around doors, windows, and ventilation points, making indoor environments vulnerable to pollution despite efforts to seal off outdoor air.

Prof Dr Victor Hoe, an occupational health and public health medicine specialist at Universiti Malaya, explained that while staying indoors provides some protection, most standard homes are not airtight. “Indoor environments offer protection, but they can quickly become secondary sources of pollution,” he said. He noted that PM2.5 particles are much smaller than a human hair, which measures approximately 70 micrometres in diameter, allowing them to bypass typical building seals. Dr Hoe also pointed out that everyday indoor activities—such as cooking, burning incense, lighting mosquito coils, and using air fresheners or paints—can contribute to indoor air pollution. He cautioned that conventional split air-conditioners only cool and recirculate air and their filters are generally designed to trap larger dust particles, not PM2.5.

Highlighting the health implications, Prof Dr Sharifa Ezzat Wan Puteh from Universiti Kebangsaan Malaysia stressed the importance of clean air for vulnerable populations, including children, the elderly, and people with heart or respiratory illnesses. She noted that children are especially susceptible as their developing lungs and higher breathing rates increase their exposure relative to body weight. PM2.5 exposure can lead to symptoms such as coughing, sore throats, rhinitis, breathing difficulties, and eye irritation. She further noted that indoor contaminants like dust, mold, and chemicals released from furniture can worsen health risks. Dr Sharifa recommended the use of air-conditioning units equipped with High-Efficiency Particulate Air (HEPA) filters, which are more effective in capturing fine particles compared to standard units.

Public health medicine specialist Datuk Dr Zainal Ariffin Omar added that the degree of protection afforded by closed windows depends on the home’s construction. He stated that indoor PM2.5 levels with closed windows typically range from 55% to 60% of outdoor concentrations, but this figure can vary widely between 30% and 100%. For households without air purifiers, Dr Zainal emphasized prioritizing the control of indoor pollution sources. He advised residents to avoid smoking or vaping indoors, limit activities that produce smoke, and use kitchen exhaust systems during cooking to reduce indoor particulate levels.

Together, these experts indicate that while closing windows can reduce some outdoor pollutant exposure during haze events, it is insufficient to fully protect indoor air quality from fine particulate matter. Managing both outdoor infiltration and indoor pollution sources is necessary to safeguard health, particularly for at-risk groups.