Washington was briefly the most polluted major city in the world, according to IQAir data, after a massive Independence Day fireworks show touted by the Trump administration as "awesome."

Pyrotecnico, the company hired by the White House's Freedom 250 organization, had set a goal of breaking a world record by launching 850,000 fireworks, though experts had warned that setting off so many in just 40 minutes could pose serious health risks.

July 4 fell during a record-breaking heat wave that upended the parades, block parties and barbecues that traditionally mark the day the United States celebrates its break from Britain.

The display of colorful explosives, intended as the day's climax, was delayed by over an hour due to a thunderstorm, finally kicking off just before midnight.

Washington's pollution monitors began registering spikes around 8pm on Saturday, signaling local fireworks displays. But levels of fine particulate matter rose dramatically during the main show, leaving people watching downwind with an obscured view as smoke came billowing their way.

Fine particle pollution -- defined as smoke and particles 2.5 micrometers in diameter that can penetrate deep into the lungs, causing acute injury but also long-term disease -- peaked at over 200 micrograms per cubic meter at one monitoring site, city-collected data reviewed by AFP showed.

The Environmental Protection Agency's (EPA) 24-hour standard caps exposure at 35 micrograms per cubic meter.

"Code Purple" alerts, denoting air quality considered very unhealthy for everyone, not just at-risk groups, blanketed much of the city as well as the wider region, including neighboring Virginia and Maryland.

And according to data compiled by Swiss company IQAir, which relies on a variety of public and private sources, Washington was the world's most polluted major city between around 3am to 5am on Sunday July 5.

It is currently in 41st position, with Jakarta, Kinshasa and Addis Ababa occupying the first three spots.

- 'Fireworks are awesome' -

Asked for comment, EPA press secretary Brigit Hirsch told AFP: "Fireworks are awesome and we hope everyone enjoyed the incredible shows in and around our nation's capital to celebrate 250 years of America."

The EPA under Trump's second term has touted its deregulatory actions and advocated on behalf of polluting industries.

Heavy rain helped clear the smoke, and conditions had returned to normal by around noon on Sunday, amounting to what scientists called a lucky break.

"We dodged a bullet. It was very bad, but it could have been substantially worse," Russell Dickerson, an atmospheric scientist at the University of Maryland, told AFP.

Ahead of the event, Pyrotecnico told AFP they were aiming to break the current world record for the largest firework display, which was set on New Year's Day 2016 in the Philippines with around 811,000 fireworks.

But it wasn't clear if they had achieved it.

A Guinness World Records spokesperson confirmed to AFP it had received an application from the Washington event.

"At this stage, we are not able to confirm whether the record has been achieved. We will only be able to do so once the evidence has been submitted and fully reviewed by our Records team."