def _apply_epa_correction(pm, rh):
    """Applies the EPA calibration to Purple's PM2.5 data.

    We floor it to 0 since the combination of very low pm2.5 concentration
    and very high humidity can lead to negative numbers.
    """
    return max(0, 0.534 * pm - 0.0844 * rh + 5.604)