When does pollen season hit your city?
City-specific pollen calendars, fungal spore windows, and the plant species most likely to trigger allergies — built from Indian aerobiological research.
Why India's pollen & spore seasons are different
Unlike Europe or North America — where a single spring tree-pollen season dominates — India's diverse climate and geography create two distinct annual pollen peaks in most cities: a spring peak (Feb–April, driven by tree pollination) and a post-monsoon peak (Aug–October, dominated by weeds and grasses).
India's monsoon is the great pollen interruptor: the June–August rains wash pollen from the air and suppress flowering, creating a natural low season. But as humidity drops post-monsoon, a fresh wave of weed and grass pollination begins — a second peak that receives less public attention than the spring season.
Thunderstorm asthma risk in India. Thunderstorms — common during the pre-monsoon, monsoon onset, and retreating monsoon periods — can trigger sudden and severe asthma exacerbations in pollen- or spore-sensitised individuals. High humidity, lightning, and strong winds cause pollen grains and fungal spores to rupture — particularly Alternaria and Cladosporium — into tiny sub-pollen particles and respirable fragments that penetrate deep into the lower airways. These microscopic particles are far more likely to reach the lungs than intact pollen grains. In cities like Delhi, Bengaluru, and Kolkata, where high pollen and spore loads often coincide with convective storms, this phenomenon can lead to sharp spikes in emergency visits. Sensitised individuals should stay indoors with windows closed during sudden thunderstorms and keep HEPA purifiers running.
Urban pollution makes pollen more dangerous, not just more abundant. Global research — including work by Indian researchers at Delhi/CSIR on Ricinus communis — has established that high NO₂ and ozone levels chemically modify pollen protein structure through nitration, increasing IgE-binding and making the same pollen grain trigger stronger immune responses. For Delhi and IGP cities with chronic high NO₂ and PM₂.₅, this means allergic responses may worsen even without any increase in pollen counts.
The monsoon is not an allergy-free window. While rain washes pollen from the air effectively, it simultaneously raises humidity to levels that drive outdoor fungal spore counts sharply upward — particularly Cladosporium and Aspergillus fumigatus.
Congress Grass (Gajar Ghas)
Parthenium hysterophorusAccidentally introduced from USA in 1956 via PL-480 wheat. Now covers 14+ million hectares of Indian farmland, roadsides, and vacant urban land. Produces up to 624 million pollen grains per plant. In Bengaluru, a 1991 IISc study found 42.5% of respiratory allergy clinic patients tested positive for Parthenium — one of the highest single-pollen sensitisation rates reported globally at the time; a 2024 Bengaluru Allergy Centre study found 7.52% in a different clinic population, reflecting methodological differences. Climate note: at current atmospheric CO₂ levels (~400 ppm), the potency of Parthenium's primary allergen (parthenin) is already 49% higher than mid-20th century levels — meaning even the same amount of pollen now triggers stronger reactions than it did decades ago. Active March–November, with a broad peak in late summer to post-monsoon (especially August–October).
Mesquite (Pahadi Keekar)
Prosopis julifloraIntroduced in the late 19th century for land rehabilitation. Now a dominant roadside tree across peninsular India. Its pollen has 16 identified allergenic components — among the highest numbers reported for any tree pollen. Particularly significant in Chennai (peaks Feb–May) and Bengaluru.