Yesterday we had a balmy 43.5 °C day here in Melbourne. By the afternoon, it was about 30 °C in my apartment, with windows closed (quite well weatherproofed) and no air conditioning. I wasn't cooking, and the only electrical appliances operating were the fridge and several computers plus network gear.
At this time, the indoor particulate reading was extraordinarily high: PM2.5 was about 40, and the particulate count for PM0.3 was about 12,000. Normal figures here are more like 4 and 500, respectively. Running an air cleaner (supposedly "true HEPA" rated) for a few hours got these numbers down by a factor of five, but only opening the windows late in the evening cleared the air.
The apartment is ancient and hasn't seen a lick of paint in over a decade, no new carpets or furniture, all the electrical stuff that was running is a few years old, and there's no gas supply.
What could be the sources of these particulates?
Edit: John B and Habropoda point out that the particulate levels were astronomical yesterday due to the NSW bushfires. Graph in Habropoda's post below.