Marketers spent $479 million to advertise on podcasts in the U.S. in 2018, a 53 percent increase over 2017, according to the Third Annual Podcast Ad Revenue Study by the Interactive Advertising Bureau (IAB) and professional services firm PwC.

The report surveyed top U.S. revenue-producing podcast companies. It  projects that ad revenues on podcasts will top $1 billion in 2021 based on market trends and surveys returned by top U.S. podcast companies.

The quality and variety of content offered by podcast publishers are helping drive growth in audience size and in the amount of time people spend listening, said Jennifer Lane, IAB audio industry lead, in a press release.

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Podcast listening is up 7 percent in one year. More than half of the population age 12 and up has listened to a podcast. Podcast listeners respond well to ads and score high marks for engagement with ads and responsiveness, according to the report.

“The tremendous growth in podcast ad spending is further evidence that podcasts continue to deliver strong results for both brand and direct response advertisers,” said Mark McCrery, CEO of Authentic and Podtrac.  “Spending by content category also maps closely to the podcast categories with the largest audiences, and with more and more places to discover podcasts, the medium is on a strong growth path.”

Marlon Nichols, who co-founded the culturally-themed fund Cross Culture Ventures, said he expects the podcasting space to continue experiencing explosive growth.

“Brands are finding that they can target consumers with greater specificity and that tracking their ad dollars in the space is far more transparent,” Nichols said in an earlier Moguldom report.