When the total ambient PM_2.5 levels are several-fold higher than the recommended limit, it may be important to study the distributions of different sizes of particulate matter (PM). Here, we assess the distributions of various sizes of total PM_2.5 for 12 months (on a monthly basis) in New Delhi, India. Importantly, we found that ultrafine particles (i.e., particles <0.5 μm) contribute significantly to total PM_2.5. PM_<0.25 were the most cytotoxic particles to human lung epithelial cells in all the 12 months. In addition, PM_<0.25 were associated with significantly higher cytotoxicity per unit mass compared to other size fractions constituting PM_2.5. For any given size of PM, the amount of reactive oxygen species (ROS) generated per unit mass is higher for the month of March as compared to that for the rest of the months in the year. The higher ROS generations for all sizes of PM collected in the month of March was not explained by differences in their metal content values. Our data suggests the lack of correlation between total PM_2.5 levels and the highly cytotoxic PM_<0.25. In summary, this work establishes the need for policy changes to routinely monitor PM_<0.25 and the necessity to establish exposure limits for PM_<0.25, especially when the total PM_2.5 levels are breached.

当总的PM _2.5水平比建议的限值高出几倍时，研究不同大小的颗粒物（PM）的分布可能很重要。在这里，我们评估了印度新德里为期12个月（每月）的各种总PM _2.5的分布。重要的是，我们发现超细颗粒（即<0.5μm的颗粒）对总PM _2.5的贡献很大。在所有的12个月中，PM _<0.25是对人肺上皮细胞最具细胞毒性的颗粒。此外，与构成PM _2.5的其他大小组分相比，PM _<0.25与单位质量的细胞毒性显着相关。对于任何给定尺寸的PM，3月月份的单位质量产生的活性氧（ROS）数量均高于一年中其余月份的数量。3月份收集的所有尺寸PM的较高ROS生成量不能通过其金属含量值的差异来解释。我们的数据表明总PM _2.5水平和高度细胞毒性PM _<0.25之间缺乏相关性。总而言之，这项工作确定了需要更改政策以常规监测PM _<0.25的必要性，以及确定PM _<0.25的暴露极限的必要性，尤其是当PM _2.5总量被破坏时。