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*Disclaimer: HeyAllergy® provides all clinical services furnished to patients in California, Florida, Texas, Illinois, Nevada, Pennsylvania. In New York, clinical services are furnished by Hey Allergy Medicine.
Real-time pollen data for Ontario — updated daily.
Ontario's position in the western Inland Empire exposes residents to a diverse and intense tree pollen season. Oak is the dominant tree allergen, with coast live oak and valley oak producing massive pollen loads from March through May that combine with the region's notorious air quality issues to create a compounded respiratory burden. Mulberry trees, widely planted as ornamentals throughout Ontario's residential neighborhoods, are among the most prolific pollen producers in Southern California — a single male mulberry can release billions of pollen grains during its March through April peak. Olive trees, common in older Ontario neighborhoods and commercial landscaping, produce highly allergenic pollen from April through June. Ash, sycamore, and walnut trees contribute heavy spring pollen. Cypress and juniper begin pollinating as early as December and continue through March, giving Ontario tree pollen exposure even in winter. Eucalyptus, planted extensively as windbreaks throughout the Inland Empire, adds year-round low-level pollen. The San Gabriel Mountains to the north support pine, cedar, and mixed conifer forests whose pollen drifts down into the valley floor on downslope winds.
Grass pollen is a prolonged and intense concern in Ontario. Bermuda grass is the dominant lawn and landscaping species throughout the Inland Empire, producing copious wind-dispersed pollen from April through October — a seven-month season enabled by Southern California's warm climate and irrigated landscapes. Ryegrass, used in winter overseeding of lawns and athletic fields, adds pollen from March through June. Fescue and bluegrass contribute to the spring and early summer mix. Ontario's extensive park system (John Galvin Park, Cucamonga-Guasti Regional Park, Creekside Park), golf courses (Whispering Lakes, The Links), and the vast stretches of irrigated median strips and roadside landscaping along Euclid Avenue, Fourth Street, and the Haven Avenue corridor create distributed grass pollen sources throughout the city. The Ontario Mills mall area and Ontario International Airport, surrounded by maintained grass and landscaping, add to the regional pollen load. Unlike cities with cold winters that kill grass, Ontario's mild temperatures allow grass to remain partially active year-round, with brief pollination events possible even in January during warm spells.
Ragweed is the primary fall weed allergen in Ontario, pollinating from August through November with peak concentrations in September and October. Sagebrush, native to the Inland Empire's semi-arid landscape, releases highly allergenic pollen from late summer through fall — the foothills north of Ontario along the San Gabriel Mountains harbor extensive sagebrush communities whose pollen drifts into the valley on Santa Ana winds. Russian thistle (tumbleweed), an iconic Inland Empire weed, produces pollen from July through October and thrives in disturbed soils around construction sites, vacant lots, and the rapidly developing areas throughout western San Bernardino County. Saltbush, pigweed (amaranth), and lamb's quarters add to the fall weed burden. Mustard weed, which covers Inland Empire hillsides in spring, contributes earlier-season weed pollen. The explosive warehouse construction boom across Ontario and surrounding communities continuously creates acres of disturbed soil — ideal habitat for invasive weeds that produce concentrated pollen loads.
Ontario's most distinctive allergen challenge isn't pollen — it's the intersection of biological allergens with the worst air quality in the United States. Ontario was ranked the #1 most polluted city in America for fine particulate matter (PM2.5) by IQAir. The Inland Empire consistently ranks first nationally for ozone pollution according to the American Lung Association. This pollution doesn't just cause respiratory symptoms on its own — it amplifies allergic responses by inflaming airways, making them more reactive to pollen, mold, and other allergens. Diesel exhaust from thousands of trucks serving Ontario's massive warehouse corridor (over 1 billion square feet of warehouse space in the Inland Empire) is responsible for approximately 70% of cancer risk from air pollution in the region. The San Gabriel and San Bernardino Mountains trap pollutants in the valley like a bowl, and prevailing westerly winds push Los Angeles's emissions directly into Ontario. Mold thrives in irrigated landscapes despite the semi-arid climate. Dust mites persist in indoor environments year-round. Pet dander and cockroach allergens are constant indoor triggers.
While pollen levels are relatively low, Ontario's allergy season never truly stops. Cypress and juniper trees begin pollinating as early as December, with counts rising through February. Winter temperature inversions trap pollutants close to the ground, creating some of the year's worst air quality days — cold, stagnant air sitting in the valley bowl formed by the San Gabriel and San Bernardino Mountains holds diesel exhaust, ozone precursors, and particulate matter at ground level. This chronic air pollution exposure keeps airways inflamed and primed for allergic reactions even when pollen is minimal. Ryegrass begins pollinating in late February. Dust and indoor allergens remain active. Santa Ana wind events (hot, dry offshore winds from the desert) can occur, stirring dust, mold, and particulates. Severity: Moderate (pollution-driven).
Ontario's most intense allergy period. Oak, mulberry, ash, sycamore, and olive trees produce massive pollen loads simultaneously. Mulberry pollen peaks in March–April, coating cars and outdoor surfaces. As temperatures rise, ozone formation accelerates — sunlight converts vehicle and industrial emissions into ground-level smog that compounds pollen-driven symptoms. Bermuda grass pollen begins its spring surge. The combination of peak tree pollen with rising ozone creates a dual respiratory assault that is uniquely severe in the Inland Empire. Warehouse truck traffic generates continuous diesel particulate exposure. Severity: Very High.
Tree pollen declines but grass pollen reaches peak intensity. Bermuda grass produces maximum loads across Ontario's irrigated landscapes. Rising temperatures (regularly exceeding 95°F by June) drive ozone to dangerous levels — the Inland Empire's ozone season runs from May through October. The heat, combined with stagnant air trapped by surrounding mountains, creates the worst sustained air quality period. Olive trees continue pollinating into June. Early weed pollen emerges. Indoor environments become critical as residents seal homes for air conditioning, concentrating dust mites and indoor mold. Severity: High (grass pollen plus severe ozone).
Summer brings Ontario's most extreme conditions. Temperatures routinely exceed 100°F, driving ozone concentrations to their annual peak — the Inland Empire records more unhealthy ozone days than anywhere else in the nation during this period. Ragweed begins pollinating in August. Russian thistle and sagebrush add weed pollen. Grass pollen continues at moderate levels. Wildfire smoke from fires in the San Gabriel Mountains, Angeles National Forest, or surrounding regions can blanket the Inland Empire for days or weeks, adding fine particulate matter to an already overwhelmed air quality environment. Santa Ana wind events in late summer stir dust and distribute allergens across the valley. Severity: Very High (ozone and emerging weeds).
Ragweed pollen reaches its annual peak across the Inland Empire. Sagebrush and Russian thistle continue producing heavy weed pollen. Santa Ana winds — hot, dry offshore winds that blow from the desert through mountain passes — peak in October and November, dramatically stirring dust, pollen, and mold spores while also creating extreme fire weather conditions. Wildfire smoke remains a significant risk. Ozone levels begin declining as temperatures moderate but remain elevated. The combination of peak weed pollen, Santa Ana wind events, and persistent air quality issues makes fall a challenging season. Severity: Very High.
Weed pollen declines after the first significant cooling in November. This is Ontario's closest approach to allergy relief — but it's compromised by winter temperature inversions that trap pollutants in the valley. Cold, still air combined with continued diesel truck traffic from the warehouse corridor creates elevated PM2.5 levels. Cypress and juniper begin their winter pollination cycle in December. Indoor allergens — dust mites, mold, pet dander — persist in sealed, heated homes. Santa Ana winds can occur through December, stirring dust and creating wildfire risk even in winter. Severity: Moderate (the best Ontario offers, still compromised by air quality).
Ontario's allergy challenges go far beyond pollen. The Inland Empire has the worst ozone pollution in the nation, and Ontario specifically was ranked #1 for PM2.5 particulate pollution. Diesel exhaust and ozone inflame your airways, making them hypersensitive to pollen, mold, and other allergens that might cause minimal symptoms in a city with clean air. This means your allergic responses in Ontario are amplified by pollution — even moderate pollen counts can trigger severe symptoms when combined with poor air quality. Monitor both pollen counts AND the Air Quality Index (AQI) daily. On high-ozone days (common May through October), minimize outdoor exposure regardless of pollen levels.
Santa Ana winds are hot, dry offshore winds that blow from the desert through the Cajon Pass and other mountain corridors into the Inland Empire. These events, most common from October through March, create multiple allergy hazards simultaneously: they stir massive amounts of dust and particulates from disturbed construction and warehouse sites, redistribute pollen and mold spores across the valley, dramatically lower humidity which dries out nasal passages, and create extreme fire weather that can lead to wildfire smoke events. During Santa Ana conditions, keep windows sealed, run HEPA air purifiers, and use saline nasal spray to counteract the dry air.
In Ontario, indoor air quality management isn't optional — it's essential. The combination of outdoor pollen, diesel particulates, ozone, and periodic wildfire smoke means your home's air filtration system is your primary defense. Use MERV 13+ filters in your HVAC system and change them monthly during peak pollution and pollen seasons (March through October). Add standalone HEPA air purifiers in bedrooms and living areas. Keep windows closed during ozone alerts, Santa Ana events, and high pollen days. Consider a portable air quality monitor to track indoor PM2.5 levels — you may be surprised how quickly outdoor pollution infiltrates your home.
Ontario is the epicenter of the Inland Empire's warehouse and logistics industry, with thousands of diesel trucks operating daily along the I-10, I-15, and Highway 60 corridors. If you live near warehouse clusters (particularly along Milliken Avenue, Haven Avenue, or near Ontario International Airport), your diesel particulate exposure is among the highest in the nation. Diesel exhaust doesn't just cause its own respiratory symptoms — it primes your immune system for stronger allergic reactions to pollen and mold. If you commute along these corridors, use your car's recirculating air setting and consider a cabin air filter upgrade. At home, HEPA filtration becomes even more critical.
The San Gabriel Mountains, Angeles National Forest, and surrounding wildland areas are prone to wildfires, particularly during Santa Ana wind events from October through January and during peak fire weather in summer. When wildfires occur, smoke can blanket the Inland Empire for days or weeks, sending PM2.5 levels to hazardous categories. Wildfire smoke contains fine particles that penetrate deep into lungs and dramatically worsen allergic and asthmatic symptoms. Stock N95 or KN95 masks for outdoor use during smoke events, keep HEPA purifiers ready, and have a plan for staying indoors during the worst smoke days. Check AirNow.gov daily during fire season.
Ontario's combination of prolonged pollen seasons, the nation's worst air pollution, Santa Ana wind events, and periodic wildfire smoke creates a respiratory environment that overwhelms seasonal antihistamine strategies. If you've been cycling through over-the-counter medications without lasting relief — especially if your symptoms intensify during ozone season or Santa Ana events — your immune system may benefit from a targeted approach. Comprehensive allergy blood testing can identify your specific Inland Empire triggers, enabling personalized sublingual immunotherapy that builds tolerance at the root cause level.
March through April is typically the most challenging period due to peak oak and mulberry pollen combined with rising ozone levels. July through October brings severe ozone pollution, ragweed pollen, Santa Ana wind events, and wildfire smoke risk. However, Ontario's year-round air quality issues mean respiratory symptoms can be triggered in any month.
Oak, mulberry, and olive tree pollen (spring), Bermuda and ryegrass (spring through fall), ragweed and sagebrush (fall), and mold spores are the top outdoor allergens. Air pollution — including diesel particulates, ozone, and periodic wildfire smoke — amplifies allergic responses to all biological allergens. Indoor triggers include dust mites, mold, cockroach, and pet dander year-round.
Ontario faces a unique combination of intense pollen seasons and the worst air quality in the United States. The Inland Empire consistently ranks #1 nationally for ozone pollution, and Ontario was ranked #1 for PM2.5 particulate matter. Mountains surrounding the valley trap both pollutants and allergens, while prevailing westerly winds push Los Angeles emissions into the region. Diesel trucks from over 1 billion square feet of warehouse space add continuous particulate pollution that inflames airways and amplifies allergic responses.
Effectively yes. Cypress and juniper pollinate in winter, oaks and mulberry peak in spring, grass runs spring through fall, ragweed and sagebrush dominate fall, and air pollution plus indoor allergens persist every day. November through mid-February offers the lowest pollen, but winter temperature inversions create poor air quality that keeps airways inflamed.
Yes. HeyAllergy provides telemedicine appointments with board-certified allergists licensed in California. Book a virtual consultation, have allergy blood tests ordered to a convenient San Bernardino County lab, and start personalized treatment — all from home. No waitlist, fast appointments available.
HeyPak allergy drops use sublingual immunotherapy (SLIT) to gradually desensitize your immune system to your specific Inland Empire triggers — whether oak, mulberry, Bermuda grass, ragweed, dust mites, or mold. You place customized drops under your tongue daily at home. Most patients see improvement within 3–6 months, with 3–5 years recommended for lasting relief.
HeyAllergy accepts Medicare and most major PPO health plans, including United Healthcare, Anthem Blue Cross, Blue Shield, Cigna, Aetna, Humana, Oscar, and Tricare. Contact your insurance provider with Tax ID: 85-0834175 to confirm your specific telemedicine coverage.
Significantly. Diesel exhaust and ozone don't just cause respiratory symptoms on their own — they inflame airways and make them hypersensitive to pollen, mold, and other allergens. Research shows that pollution exposure amplifies allergic responses, meaning moderate pollen counts in Ontario can trigger severe symptoms that the same pollen levels wouldn't cause in a city with clean air. This pollution-allergen multiplier effect is a defining feature of living in the Inland Empire.
Ontario, California occupies a unique and troubling position in America's allergy landscape. This city of approximately 180,000 residents sits at the western gateway of the Inland Empire — the sprawling metropolitan region east of Los Angeles that encompasses San Bernardino and Riverside Counties. What makes Ontario's allergy environment distinctive isn't just its pollen profile, which mirrors much of Southern California. It's the unprecedented intersection of biological allergens with the worst air quality in the United States. Ontario was ranked the #1 most polluted city in America for fine particulate matter (PM2.5) by IQAir, and the broader Inland Empire consistently ranks first nationally for ozone pollution according to the American Lung Association. For the estimated 50+ million Americans who suffer from allergies, living in Ontario means experiencing allergic symptoms amplified by a pollution burden that has no equal in the continental United States.
Ontario was founded in 1882 by George and William Chaffey, Canadian engineers who designed an innovative irrigation system that transformed the semi-arid landscape into productive agricultural land. Named after the Chaffey brothers' home province in Canada, the city grew as a center for citrus farming and vineyards. Today, Ontario is the economic engine of the western Inland Empire, anchored by Ontario International Airport, the Ontario Mills shopping center, and — most significantly for air quality — one of the largest warehouse and logistics concentrations in the world. Ontario's convenient location at the intersection of Interstate 10, Interstate 15, and Highway 60, combined with relatively affordable land compared to coastal Los Angeles, has made it the epicenter of Southern California's goods movement industry.
Ontario's air quality crisis begins with geography. The city sits on the valley floor at the base of the San Gabriel Mountains to the north, with the San Bernardino Mountains to the northeast and the hills separating the Inland Empire from the Los Angeles Basin to the west. This mountain-ringed valley functions as an enormous bowl that traps pollutants. Prevailing westerly winds push emissions from the Los Angeles Basin — home to millions of vehicles, the ports of Los Angeles and Long Beach, and extensive industrial activity — directly eastward into the Inland Empire, where mountains block further dispersion. The trapped pollutants cook in Southern California's abundant sunshine, producing ground-level ozone (smog) at concentrations that consistently exceed federal health standards.
For allergy sufferers, this geography creates a compounding effect. Pollen released by trees, grasses, and weeds across the valley floor has nowhere to go — it concentrates in the same trapped air mass as ozone, diesel particulates, and industrial emissions. The resulting mixture is more harmful than any single component alone. Research shows that air pollution inflames respiratory airways, making them hyperreactive to allergens that might cause minimal symptoms in clean-air environments. In Ontario, a moderate oak pollen day combined with elevated ozone can produce severe allergic symptoms that the same pollen count wouldn't trigger in a coastal city with clean ocean breezes.
The Inland Empire contains over one billion square feet of warehouse space — a staggering concentration of logistics infrastructure that processes an estimated 40% of all goods entering the United States through Southern California's ports. Ontario sits at the heart of this warehouse corridor. Thousands of diesel trucks operate daily along the I-10, I-15, and Highway 60 corridors that converge in Ontario, hauling containers from the ports of Los Angeles and Long Beach to warehouse distribution centers, then redistributing goods to destinations across the country.
Diesel exhaust is responsible for approximately 70% of the total cancer risk from air pollution in the Inland Empire. Near Ontario's warehouse clusters, cancer risk reaches the 95th percentile — 95% higher than the rest of the Los Angeles air basin. But the impact on allergies goes beyond cancer risk. Diesel particulate matter (PM2.5) penetrates deep into lung tissue, triggering inflammatory responses that sensitize airways to biological allergens. Residents living near warehouse corridors — particularly along Milliken Avenue, Haven Avenue, Archibald Avenue, and the areas surrounding Ontario International Airport — face the most intense diesel exposure. The trucks don't stop at night or on weekends; the logistics industry operates 24/7, creating continuous particulate pollution that keeps respiratory systems in a state of chronic inflammation.
Santa Ana winds are hot, dry offshore wind events that blow from the desert interior through mountain passes — including the Cajon Pass northeast of Ontario — into the Inland Empire and coastal Southern California. These events, most common from October through March, fundamentally alter Ontario's allergen environment. Santa Ana winds stir massive quantities of dust from the valley's extensive disturbed land — construction sites, vacant lots, warehouse development areas, and agricultural remnants. They redistribute pollen and mold spores that had settled, sending them airborne again across the region. The dramatically low humidity (sometimes below 10%) dries out nasal passages, stripping away the protective mucus layer that helps filter allergens.
Santa Ana events also create extreme fire weather conditions. The San Gabriel Mountains and Angeles National Forest, visible from virtually anywhere in Ontario, are fire-prone landscapes that periodically burn in large wildfires. When fires occur during Santa Ana conditions, smoke blankets the Inland Empire for days or weeks, sending PM2.5 to hazardous levels. Wildfire smoke contains ultrafine particles that penetrate deeper into lungs than typical urban pollution, triggering severe respiratory reactions in allergy and asthma sufferers. The combination of a Santa Ana wind event, wildfire smoke, and existing warehouse pollution creates what residents describe as some of the worst breathing conditions imaginable — and these events recur with regularity every fire season.
Ontario's biological allergens — oak, mulberry, olive, ash, and sycamore trees; Bermuda, ryegrass, and fescue grasses; ragweed, sagebrush, and Russian thistle weeds — are broadly similar to other Southern California cities. What makes Ontario's pollen experience different is the amplification effect of the region's air pollution. Every pollen grain that enters airways already inflamed by ozone and diesel particulates produces a stronger allergic response than it would in clean air. This pollution-allergen multiplier means that Ontario residents may develop allergies they never had before, or find that mild allergies they managed easily in other cities become debilitating in the Inland Empire.
Mulberry trees deserve special attention in Ontario. Widely planted throughout older Inland Empire neighborhoods for shade, male mulberry trees are among the most prolific pollen producers in the plant world. A single tree can release billions of wind-dispersed pollen grains during its March–April peak. Several California cities, including Las Vegas and Tucson, have banned new mulberry plantings specifically because of their allergenic impact. Ontario's existing mulberry canopy continues to produce enormous pollen loads each spring, and the pollen's small size allows it to remain airborne for extended periods — particularly in the trapped valley air.
Ontario's natural climate is semi-arid, receiving only about 15 inches of rainfall annually. In theory, a dry climate should suppress mold growth and reduce allergen diversity. In practice, the extensive irrigation infrastructure that made Ontario's founding possible — and that continues to support its lawns, parks, golf courses, and commercial landscaping — creates artificial moisture environments throughout the city. Irrigated landscapes support year-round grass growth (and pollination), sustain mold colonies in mulch beds and soil, and maintain humidity levels in yards and parks that would never exist naturally in this climate.
The result is a city where outdoor mold and grass pollen persist at levels more typical of humid climates, while the surrounding semi-arid landscape contributes its own unique allergens: sagebrush from foothill communities, Russian thistle from disturbed desert soils, and dust from unpaved surfaces during dry periods. Ontario residents face allergens from both irrigated urban environments and native desert ecosystems — a dual exposure profile that comprehensive allergy testing can untangle.
Ontario's combination of intense pollen seasons, the nation's worst air pollution, Santa Ana wind events, and periodic wildfire smoke creates a respiratory environment that overwhelms typical allergy management strategies. Taking antihistamines during oak season while breathing diesel particulates and ozone is like applying a bandage while the wound continues to be aggravated. Many Ontario residents cycle through medications for years without finding lasting relief because they're treating symptoms rather than addressing the underlying immune sensitivity that pollution is continuously amplifying.
HeyAllergy offers Ontario and Inland Empire residents convenient telemedicine access to board-certified allergists and immunologists who understand the unique challenges of living in America's most polluted metropolitan area. Through a secure video consultation, your allergist can evaluate your symptoms, order comprehensive blood allergy testing at a convenient local San Bernardino County lab, and develop a personalized treatment plan that accounts for the pollution-allergen interaction. For patients who qualify, HeyPak sublingual immunotherapy drops can be customized to your specific test results and the allergens endemic to the Inland Empire, delivered directly to your Ontario home, and taken daily under the tongue. Most patients notice improvement within 3–6 months, with 3–5 years of treatment recommended for lasting relief. Starting at $47/month, HeyPak offers a path toward immune resilience in an environment that demands it — no needles, no clinic visits, no waitlist.
