The frequency and severity of wildfires are increasing throughout much of the world, which pose risks to human health through inhalation of airborne particulate matter less than 2.5 µm in diameter (PM_2.5) in the wildfire smoke. PM_2.5 may originate from ash and soil nanoparticles (NPs), which are often associated with toxic metals such as Cr(VI)  formed by the oxidation of Cr(III) during wildfires.  PM_2.5 is a general term used for any particulate < 2.5 µm; however, a wide variety of particulates with different sizes and physical and chemical properties can be formed in this size range, which are not well studied. The health risks due to exposure to PM_2.5 can be different for various PM_2.5 phases with different sizes, metal content, and crystal chemistries and structures. We studied the size and structure of fire-induced soil NPs and their association with Cr(VI). Soils with different mineralogy (serpentine, greenstone, sandstone, and chert) were collected from the Jasper Ridge Biological Preserve (San Mateo County, CA) and heated to temperatures ranging from 200 to 800 °C for 2 h. The samples were characterized using X-ray diffraction, X-ray fluorescence spectrometry (XRF), scanning and transmission electron microscopy (EM), synchrotron X-ray absorption spectroscopy, and wet chemistry experiments. The concentration of Fe and Cr showed a positive correlation in all the samples, with the highest concentration in serpentine soils; 117 ± 4 mg/g for Fe and 3.90 ± 4 mg/g for Cr.  The primary Fe minerals in all the samples were magnetite which transformed to hematite with increasing temperature. Using Energy dispersive X-ray spectroscopy of EM, we found that Fe (hydr)oxide NPs are associated with Cr, which increased with burning. More than 2.5% of Cr(III) was transformed to exchangeable Cr(VI) with burning up to 600 ^∘C in all the samples. The exchangeable Cr(VI) concentration was slightly higher in chert samples, attributed to their lower concentration of Fe (hydr)oxide NPs. Additionally, EM showed that the size of particles in fresh and burned samples ranges between 5-500 nm, with most particles being less than 100 nm. Our results showed that the size of airborne PM originating from soils could be much smaller than 2.5 µm, which increases their association with wildfire-induced toxic metals such as Cr(VI), and assist their penetration into the human lung and beyond. This information is critical to inform targeted policies and interventions for mitigating respiratory health risks to firefighters, farm workers, and local communities who are disproportionately impacted by wildfire smoke.