The mercury is set to reach nearly 30C in parts of the country, more than 10C above the seasonal average of around 17C.
The heatwave is being caused by a warm front and a tropical continental air mass from Europe pushing across the country, bringing elevated temperatures and high humidity.
But forecasters were predicting international air safety standards will be broken in 31 council areas north of the Border, sparking fears that lives could be at risk.
Air pollution increases the risk of heart attacks and strokes and can affect unborn babies.
Official figures suggest nearly 2,000 deaths in Scotland every year are caused by exposure to toxic particles in the air.
Ground-level ozone is considered the biggest danger during the latest episode, with levels predicted to break World Health Organisation safety guidelines.
Scotland’s official colour-coded air pollution forecast puts today’s levels at “moderate”, but climate experts say such a rating is relatively unusual and will see World Health Organisation limits exceeded in many areas..
Ozone concentrations in Edinburgh were predicted to reach between 101 and 120 microgrammes per cubic metre, with guidelines recommending levels stay under 100 microgrammes per cubic metre.
Health advice for today’s levels recommends people with heart and lung problems such as asthma or emphysema should cut back on strenuous physical activity, particularly outdoors.
But the current warm and humid weather conditions are only part of the problem, according to environmentalists.
“Some of this air pollution will have blown in from elsewhere but it is adding to the heavy air pollution caused by traffic on our streets,” said Emilia Hanna, of campaign group Friends of the Earth Scotland.
“Regular exposure to the levels of pollution that we see day in, day out on city streets in Scotland increases the risk of people having a heart attack or stroke.
“Most disturbingly, air pollution impacts on foetal development, with newborn babies more likely to have lower birth weights if expectant mothers are exposed to air pollution throughout pregnancy.”
A spokeswoman for the Scottish Government confirmed alerts were being sent out to registered users of its air pollution messaging system. She also advised people who find traffic fumes make breathing more difficult to avoid busy streets.
Today’s temperatures were estimated to reach between 23C and 26C in most places, though some western areas could see highs of up to 29C.
Meteorologists warned of sporadic thundery downpours as the day wears on, with conditions becoming fresher into tomorrow and Friday before warming up again this weekend.