Lower atmosphere variations in the martian water vapour and hydrogen abundance during the late southern summer regional-scale dust storm from LS = 326.1°–333.5° Mars Year34 (18th-31st January 2019) and their associated effect on hydrogen escape are investigated using a multi-spacecraft assimilation of atmospheric retrievals into a Martian global circulation model. The dusty conditions led to an elevation of the hygropause and associated increase in middle atmosphere hydrogen at the peak of the dust storm, which was particularly intense in Mars Year34. This response has an important effect on water loss during this time period, quantification of which can provide a more robust calculation of the integrated loss of water through time on Mars and better insight to planetary evolution. The influx of water vapour to higher altitudes led to an associated increase in hydrogen through photolysis of water vapour and a hydrogen escape rate of around 1.0×109 cm−2s−1, meaning the late southern summer regional-scale dust storm in Mars Year34 enhanced water loss rates on Mars to levels observed during global-scale dust storms. The water loss rate during the late southern summer regional-scale dust storm, also known as a C storm, led to around 15% of the total annual water loss during only 5% of the year and was at least three times stronger than the much less dusty C storm event in Mars Year30. These results demonstrate that interannual variations in the strength of the late southern summer regional-scale dust storm must be considered when calculating the integrated loss of water on Mars, an important quantity to constrain in relation to the potential habitability of Mars.