London, Jun 13 (IANS): While outdoor pollution level appeared to drop during Covid-induced periods of lockdown, particulate matter (PM) 2.5 levels were on average 23 per cent higher indoors, according to a new study of 10 global cities.

The cities included London, Paris, Berlin, Milan, New York, Toronto, Manila, Kuala Lumpur, Bangkok and Seoul.

Particle pollution was found to be high indoors in most cities even after the lockdown had lifted.

British home appliance maker Dyson collected data during the first phases of lockdown between March and May, while post-lockdown data was collected between June and September, when many countries had loosened Covid restrictions.

As Covid restrictions now continue across the globe, higher levels of indoor particulate matter can once again be expected, the study warned.

The reason could be attributed to new habits incurred during lockdown. For example, the year 2020 saw a significant uplift in pet ownership. Pet dander, pollen and other organic material may be significant sources of both PM2.5 and PM10, which are likely increasing indoor pollution and allergen levels.

"Before the pandemic, indoor PM2.5 generated indoors by normal human functions would be spread between the home, the office and other indoor spaces. Instead, it all builds up in our homes now. Lockdown is like a perfect storm -- the growing indoor plants trend, uplift in pet ownership and increase in flower deliveries can all increase allergen levels in indoor air, when we're spending more time at home than ever," said Alex Knox, Vice-President of Engineering for Environmental Care, in a statement.

Further, actions like frying food in the kitchen can increase PM2.5 levels by five times. The combustion process while cooking releases particulates into the air, likely contributing to this increase, the study noted.

Usage of common household items like deodorants, perfumes, aerosols and cleaning sprays also increases the concentration of volatile organic compounds within our homes.

With homes now also doubling up as offices, schools and gyms, increased indoor allergens may cause greater disruption than in previous years on working, studying and exercise, as well as normal, indoor activities like sleeping and relaxing, the study maintained.

The study also emphasised that cleaning indoor surfaces and vacuuming carpets isn't always enough to prevent the inhalation of allergens and that attention should be paid to indoor air quality.