Mr. Castle’s experiences with Facebook echoed those of other small mask producers who have recently began making N95s and other medical grade masks. Largely shut out by hospital networks, they had hoped to sell their high-filtration products online, where Americans do much of their shopping. But the tech giants have not made it easy, even as scientists have urged the public to upgrade their face coverings to those that can block the tiny pathogens that cause infection.

Google and Facebook ban the sale of medical-grade masks, and Amazon limits their availability to individual shoppers — policies born during the early months of the pandemic, when hospitals were scrambling to obtain protective gear.

But some public health experts and mask manufacturers say these rules are outdated, especially given the spread of more infectious coronavirus variants and the abundance of domestically made masks that are gathering dust in warehouses across the country. The restrictions, they say, may hinder the country’s ability to limit new infections in the months before vaccinations become more widely available.