I will be traveling to areas affected by wildfire smoke.

Does anyone have recommendations for a travel air purifier with a HEPA filter?

fyi - summarized from an EPA report on wildfire smoke a summary of why breathing smoke, aka "particle pollution", is dangerous. I added bolded words.

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[...] Additionally, the size of particles affects their potential to cause health effects.
=== Human Hair === diameter 60 micrometers
For purposes of comparison, a human hair is about 60 micrometers in diameter.

Particles larger than 10 micrometers do not usually reach the lungs, but can irritate the eyes, nose, and throat.

=== PM10 === diameter 10 micrometers & smaller

Small particles with diameters less than or equal to 10 micrometers, also known as particle pollution or PM10, can be inhaled deep into the lungs. Exposure to the smallest particles can affect the lungs and heart.

=== PM2.5 === diameter 2.5 micrometers & smaller

Particle pollution known as PM10-2.5 or just PM2.5,includes both
- coarse particles, with diameters from 2.5 to 10 micrometers
- fine particles with diameters less than 2.5 micrometers

Particles from smoke tend to be very small, with a size range near the wavelength of visible light (0.4 – 0.7 micrometers). Therefore, smoke particles efficiently scatter light and impact visibility. Moreover, these particles are within the fine particle PM2.5 fraction and can be inhaled into the deepest recesses of the lung and may represent a greater health concern than larger particles.

From: the EPA guide to wildfire smoke
https://www3.epa.gov/airnow/wildfire_may2016.pdf