Abstract
Background
Knowledge of airborne pollen seasons is essential for physicians to accurately diagnose and treat patients with allergic respiratory diseases. Although the Dallas-Fort Worth metropolitan area in North Texas is home to more than 8 million residents, it lacks a published pollen calendar.
Objective
Our objectives were 3-fold: (1) determine the most common allergenic pollens in North Texas and when they are present, (2) identify how pollen concentrations have changed over time, and (3) assess how weather affects the daily pollen concentrations.
Methods
We obtained 15 years of daily pollen concentration and weather data for North Texas. Data were analyzed in R by using the AeRobiology package. The AeRobiology package was used to interpolate missing data, create heatmaps of daily pollen concentrations, and calculate the pollen seasons. We use regressions accounting for seasonal effects to determine changes over time and the effect of weather factors.
Results
In North Texas, pollen is present throughout the year. Spring-dominant Quercus and winter-dominant Juniperus constitute more than half of the total annual pollen production. Ulmus and Ambrosia make up the bulk of the fall pollen. Consistent with climate change, daily pollen concentrations have been increasing over time. Moreover, higher pollen concentrations are associated with higher maximum daily temperature and average daily wind speed.
Conclusions
These pollen calendars will help physicians in the region care for patients with allergic respiratory disease, who may present with more severe disease as pollen concentrations are increasing over time.
Key words: Pollen, pollen allergy, seasons, seasonal allergic rhinitis, Dallas-Fort Worth metropolitan area, climate change, Quercus, Juniperus, Ulmus, Ambrosia, calendar
Introduction
Texas is one of the states most affected by increases in pollen concentrations related to climate change, and for allergy sufferers, the Dallas-Fort Worth (DFW) metropolitan area in North Texas specifically is often ranked as one of the most challenging regions in the United States in which to live.1,2 Although pollen calendars have been produced for the San Antonio and Washington, DC metro areas, providing guidance to allergy providers in those areas, a pollen calendar for the DFW region has not yet been published.3,4
In this study, we have analyzed 15 years of pollen data to answer 3 key questions: What are the most common allergenic pollens in North Texas, and when are they present? How have the seasons of these allergenic pollens changed over time? and How are weather factors related to daily pollen concentrations?
The setting of this study is the DFW metroplex, a wide geographic area straddling 2 ecologic areas: the Blackland Prairies on the East, which are dominated by numerous species of grass, and the Cross Timbers and Prairies ecoregion on the West, which are forest regions dominated by the post Oak (Quercus stellata) and blackjack Oak (Quercus marilandica).5 Rainfall in DFW averages 94 cm per year, which is a bit higher than the state average but significantly lower than the average in the easternmost areas of the state. Average monthly low and high temperatures range from 2°C to 14°C in January to as high as 24°C to 35°C in August.6
Data for this study were drawn from 2 primary sources. Daily pollen concentrations were obtained from the National Allergy Bureau for the North Texas Pollen Station in DFW, which used a Burkard spore trap for the days between January 1, 2009, and December 31, 2023.7 Daily weather data were obtained from the National Oceanic and Atmospheric Administration.6
Results and Discussion
Daily pollen concentration data were nearly complete, with just 1% of data missing. The AeRobiology package in R was used to interpolate the missing data by using the moving means method, as well as to calculate and provide visualizations of the pollen seasons.8 The moving means method was chosen, as it may have lower relative errors than interpolation of missing pollen data by linear or spline regression.9 For the sake of parsimony, pollens detected inconsistently or at very low levels were combined into a category dubbed other trees (eg, Fagus, Prosopis) and other weeds (eg, Urticaceae, Cyperaceae).
To address our first question, namely, the most common allergenic pollens and the timing of their presence, we calculated pollen seasons by using a percentage criterion and the AeRobiology package. With use of procedures similar to those used by Kosisky et al, who described the pollen seasons of Washington, DC, we identified the start and end of the seasons by determining when a certain percentage of the annual pollen integral is reached.4 Like Kosisky et al,4 both a 1%/99% and a narrower 5%/95% pollen season were calculated for each taxon as well as for the peak pollen concentration date (Table I).
Table I.
Average pollen season start, peak, and end dates, 2009-2023
| Pollen | 1% start | 5% start | Peak | 95% end | 99% end | Mean APIn |
|---|---|---|---|---|---|---|
| Juniperus | Nov 2 | Dec 17 | Jan 17 | Mar 7 | Mar 27 | 25,676 |
| Ulmus (spring) | Jan 27 | Feb 2 | Feb 15 | Mar 3 | Mar 12 | 3,700 |
| Populus | Mar 9 | Mar 12 | Mar 19 | Apr 6 | Apr 14 | 820 |
| Liquidambar | Mar 11 | Mar 14 | Mar 21 | Apr 4 | Apr 12 | 129 |
| Morus | Mar 14 | Mar 17 | Mar 21 | Apr 12 | Apr 20 | 5,780 |
| Fraxinus | Feb 22 | Mar 4 | Mar 22 | Apr 10 | Apr 18 | 2,988 |
| Acer | Mar 8 | Mar 13 | Mar 22 | Apr 6 | Apr 13 | 570 |
| Betula | Mar 6 | Mar 13 | Mar 23 | Apr 17 | Apr 22 | 299 |
| Pinaceae | Feb 23 | Mar 2 | Mar 27 | May 6 | Jun 25 | 1,258 |
| Celtis | Mar 13 | Mar 17 | Mar 27 | Apr 17 | Apr 24 | 2,470 |
| Platanus | Mar 12 | Mar 15 | Mar 31 | Apr 12 | Apr 17 | 573 |
| Quercus | Mar 13 | Mar 18 | Mar 31 | Apr 10 | Apr 17 | 39,534 |
| Salix | Mar 15 | Mar 20 | Apr 1 | Apr 23 | May 4 | 903 |
| Juglans | Mar 18 | Mar 22 | Apr 8 | May 2 | May 23 | 100 |
| Rumex | Mar 7 | Mar 25 | Apr 20 | Jun 5 | Aug 15 | 328 |
| Other trees | Mar 19 | Mar 30 | Apr 28 | May 19 | May 23 | 315 |
| Carya | Apr 8 | Apr 15 | Apr 29 | May 12 | May 24 | 724 |
| Poaceae | Mar 7 | Mar 26 | May 9 | Oct 5 | Nov 3 | 3,178 |
| Other weeds | Mar 28 | Apr 7 | Jul 4 | Oct 13 | Oct 28 | 146 |
| Artemsia | Jul 10 | Jul 21 | Aug 16 | Sep 21 | Sep 29 | 146 |
| Amaranthaceae | Mar 20 | May 29 | Aug 25 | Oct 21 | Nov 21 | 399 |
| Ulmus (fall) | Aug 21 | Aug 27 | Sep 14 | Oct 3 | Oct 11 | 11,072 |
| Ambrosia | Jul 21 | Sep 5 | Sep 29 | Oct 22 | Nov 14 | 7,204 |
APIn, Annual pollen integral.
Pollen season start and end dates according to the 1%/99% and tighter 5%/95% criteria, in order of peak pollination date. Mean APIn is included for reference. Ulmus has separate listings for its spring and fall pollination periods. The majority of pollen taxa peak in the spring, with Juniper, Ulmus, and Ambrosia being important outliers. Poaceae peaks in May but continues pollen production well into the fall.
AeRobiology was also used to create a heatmap to visualize the average daily pollen concentrations in descending order of each taxon’s pollen contribution (Fig 1). As Fig 1 demonstrates, in North Texas pollen is present throughout the year, with the spring-dominant Quercus and winter-dominant Juniperus together accounting for more than half of the annual pollen production and Ulmus and Ambrosia making up the bulk of the fall pollen.
Fig 1.
Heatmap of average North Texas pollen concentrations in order of pollen abundance.
To address our second research question, namely, how pollen seasons have changed over time, we performed ordinary least square regressions in R to estimate the year-to-year change in daily pollen concentrations for each taxon (Table II). The models also included fixed effects for day of the year (not reported) to ensure that our estimates reflected the average annual increase in each pollen with adjustment for natural daily variation and that they were calculated with robust SEs to account for serial autocorrelation (for more information on fixed effects estimation in the panel data, see Allison10). Although a few taxa (Populus, Salix, Artemisia, and Rumex) had a slight downward trend, the total daily pollen concentration had an upward trend (P < .001) driven by particularly large increases in the 4 most prolific pollens: Quercus, Juniperus, Ulmus, and Ambrosia.
Table II.
Change in daily pollen concentrations per year over the study period, with increasing pollen concentrations at the top and decreasing pollen concentrations at the bottom
| Pollen | B | SE | t | P value |
|---|---|---|---|---|
| Total pollen | 10.559‡ | 1.744 | 6.056 | <.001 |
| Quercus | 6.684‡ | 1.189 | 5.62 | <.001 |
| Juniperus | 2.047∗ | 0.846 | 2.42 | .016 |
| Ulmus | 1.324 | 0.679 | 1.95 | .052 |
| Ambrosia | 0.516‡ | 0.129 | 3.993 | <.001 |
| Fraxinus | 0.161 | 0.088 | 1.821 | .069 |
| Pinaceae | 0.132∗ | 0.061 | 2.174 | .030 |
| Acer | 0.077† | 0.025 | 3.017 | .003 |
| Betula | 0.075† | 0.026 | 2.889 | .004 |
| Platanus | 0.046 | 0.027 | 1.678 | .094 |
| Morus | 0.019 | 0.235 | 0.08 | .937 |
| Amaranthaceae | 0.002 | 0.007 | 0.356 | .723 |
| Juglans | –0.0003 | 0.006 | –0.05 | .960 |
| Carya | –0.001 | 0.017 | –0.049 | .962 |
| Liquidambar | –0.006 | 0.008 | –0.797 | .426 |
| Celtis | –0.011 | 0.082 | –0.128 | .899 |
| Other weeds | –0.016 | 0.014 | –1.179 | .239 |
| Rumex | –0.034‡ | 0.009 | –4.008 | <.001 |
| Poaceae | –0.041 | 0.049 | –0.829 | .408 |
| Artemisia | –0.049‡ | 0.014 | –3.486 | .001 |
| Salix | –0.094∗ | 0.045 | –2.076 | .038 |
| Populus | –0.105† | 0.034 | –3.046 | .003 |
| Other trees | –0.165‡ | 0.028 | –5.999 | <.001 |
B, Unstandardized regression coefficient (change in daily pollen concentration per year); t, T-statistic
P ≤ .05.
P ≤ .01.
P ≤ .001.
Finally, to address our third research question, namely, the relationship between weather and pollen concentrations, we performed ordinary least square regressions estimating the relationship between the daily pollen concentration for each taxon and daily precipitation (in mm), maximum daily temperature (in °C), and average daily wind speed (in m per second) (Table III). As already stated, the models also accounted for fixed effects for year and day (not reported) and were estimated with robust SEs. We found that total daily pollen concentrations were positively correlated with both average wind speed and maximum daily temperature (P < .001). Moreover, these relationships were similar and statistically significant for almost all of the individual pollen taxa. Conversely, daily precipitation was negatively correlated with same-day pollen concentrations for most taxa but was statistically significant for only a few taxa, including Poaceae, Ambrosia, and Amaranthaceae (P < .001 for each).
Table III.
Linear regression model for the effect of daily average wind speed (m/s), precipitation (mm), and maximum temperature (C), on same day pollen concentrations
| Pollen | Wind speed (m/s) |
Precipitation (mm) |
Maximum Temperature (C) |
|||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| B | SE | t | P value | B | SE | t | P value | B | SE | t | P value | |
| Total pollen | 47.606‡ | 5.272 | 9.03 | <.001 | –0.318 | 0.751 | –0.424 | .672 | 28.294‡ | 2.167 | 13.056 | <.001 |
| Quercus | 21.281‡ | 4.096 | 5.196 | <.001 | –0.152 | 0.500 | –0.305 | .761 | 8.539‡ | 1.539 | 5.549 | <.001 |
| Juniperus | 11.138‡ | 2.299 | 4.845 | <.001 | 0.534 | 0.282 | 1.89 | .059 | 14.529‡ | 1.350 | 10.759 | <.001 |
| Ulmus | 6.895‡ | 1.688 | 4.085 | <.001 | –0.211 | 0.371 | –0.569 | .57 | 1.774‡ | 0.458 | 3.875 | <.001 |
| Ambrosia | 2.163‡ | 0.346 | 6.256 | <.001 | –0.252‡ | 0.060 | –4.181 | <.001 | 0.187 | 0.116 | 1.613 | .107 |
| Morus | 1.523∗ | 0.661 | 2.303 | .022 | –0.073 | 0.082 | –0.888 | .375 | 1.351‡ | 0.201 | 6.708 | <.001 |
| Poaceae | 1.078‡ | 0.293 | 3.68 | <.001 | –0.103‡ | 0.023 | –4.475 | <.001 | 0.118‡ | 0.032 | 3.639 | <.001 |
| Fraxinus | 0.704† | 0.269 | 2.616 | .009 | 0.002 | 0.027 | 0.079 | .938 | 0.628‡ | 0.102 | 6.153 | <.001 |
| Celtis | 0.695† | 0.252 | 2.758 | .006 | 0.05 | 0.036 | 1.385 | .166 | 0.503‡ | 0.080 | 6.313 | <.001 |
| Pinaceae | 0.116 | 0.113 | 1.03 | .304 | –0.009 | 0.019 | –0.48 | .632 | 0.154‡ | 0.043 | 3.575 | <.001 |
| Salix | 0.415† | 0.137 | 3.034 | .003 | 0.003 | 0.016 | 0.203 | .84 | 0.074∗ | 0.034 | 2.155 | .032 |
| Populus | 0.154 | 0.101 | 1.529 | .127 | –0.002 | 0.011 | –0.171 | .865 | 0.15‡ | 0.029 | 5.149 | <.001 |
| Carya | 0.166‡ | 0.050 | 3.314 | .001 | –0.019∗ | 0.008 | –2.446 | .015 | 0.073‡ | 0.013 | 5.434 | <.001 |
| Platanus | 0.192† | 0.063 | 3.036 | .003 | –0.001 | 0.009 | –0.125 | .901 | 0.05† | 0.018 | 2.745 | .007 |
| Acer | 0.313‡ | 0.093 | 3.38 | .001 | –0.017 | 0.011 | –1.502 | .134 | 0.085∗ | 0.043 | 1.966 | .05 |
| Amaranthaceae | 0.132‡ | 0.019 | 7.03 | <.001 | –0.021‡ | 0.003 | –7.006 | <.001 | –0.007 | 0.006 | –1.025 | .306 |
| Rumex | 0.091‡ | 0.027 | 3.317 | .001 | –0.004 | 0.004 | –0.916 | .36 | 0.021‡ | 0.006 | 3.594 | <.001 |
| Other trees | 0.142 | 0.099 | 1.443 | .15 | –0.022† | 0.008 | –2.635 | .009 | –0.01 | 0.014 | –0.731 | .465 |
| Betula | 0.109∗ | 0.053 | 2.071 | .039 | –0.004 | 0.009 | –0.414 | 0.68 | 0.05∗ | 0.022 | 2.304 | .022 |
| Other weeds | 0.084† | 0.030 | 2.811 | .005 | –0.004 | 0.003 | –1.384 | .167 | –0.006 | 0.007 | –0.806 | .421 |
| Artemisia | 0.104∗ | 0.048 | 2.189 | .029 | –0.009† | 0.003 | –2.934 | .004 | –0.013 | 0.010 | –1.321 | .187 |
| Liquidambar | 0.084‡ | 0.022 | 3.787 | <.001 | –0.002 | 0.002 | –1.072 | .284 | 0.034‡ | 0.005 | 6.322 | <.001 |
| Juglans | 0.024 | 0.019 | 1.266 | .206 | –0.002 | 0.001 | –1.794 | .073 | 0.01† | 0.004 | 2.641 | .009 |
B, Unstandardized regression coefficient (change in daily pollen concentration per unit change in wind speed, precipitation, and maximum temperature respectively); t, T-statistic.
P ≤ .05.
P ≤ .01.
P ≤ .001.
Our results indicate that pollen is prevalent throughout the year in North Texas, although total pollen concentration tends to be at its lowest levels in July. Multiple trees, most prolifically Quercus, pollinate in the spring. Grass pollinates at high levels in the late spring and early summer and at lower levels through the fall. Ulmus releases heavy pollen in the spring and then even heavier pollen loads in the fall, when it is accompanied by Ambrosia. Juniperus releases large amounts of pollen throughout the winter months.
An important limitation to our study is the use of a single pollen station. Daily pollen concentrations can vary significantly across an urban area, and environmental changes near the pollen collector may affect its results.11 An example of this limitation is presented in Table II, in which some pollen concentrations decreased over the study period. This could reflect a true decrease in the pollen concentration over North Texas; alternatively, it could be related to an unaccounted-for local event, such as a new parking lot or building construction.
We found that for many pollens—specifically, for the most abundant pollen producers—there has been a trend toward higher and higher pollen concentrations over time. This may portend more severe symptoms for patients experiencing allergic respiratory disease. Further, as maximum daily temperature and average wind speed were generally associated with higher daily pollen concentrations, climate change may play a further role in increased pollen exposure in the years ahead.1 North Texas has experienced a 0.3°C increase in annual average temperature per decade since 1973 and a 1.5% increase in overall precipitation per decade since 1895.12 The increased temperature is likely to bring higher pollen concentrations in the years ahead, and although precipitation is associated with decreased same-day pollen concentrations, the overall impact on annual pollen production is complex.13
This is, to our knowledge, the first publication on the pollen seasons in DFW, and it should be helpful to physicians caring for patients with respiratory allergy conditions. This information should help physicians and their patients who are sensitized to these pollens to identify the time periods during which they may be at higher risk for flares in their allergic rhinoconjunctivitis and asthma, as well as to potentially adjust their preventive therapies accordingly. Also, when immunotherapy is prescribed, this information will enable more accurate assessment of which pollen sensitizations are clinically relevant to an individual patient’s seasonal symptoms.
Key messages.
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•
Airborne pollen is present in North Texas throughout the year and is increasing over time.
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•
The pollen calendars created in this study will help allergists in the region care for their patients with allergic respiratory diseases.
Disclosure statement
Disclosure of potential conflict of interest: The authors declare that they have no relevant conflicts of interest.
Acknowledgments
We thank the American Academy of Allergy, Asthma & Immunology’s National Allergy Bureau for providing the pollen data and Dr Marie Fitzgerald and Lisa Jeter at the North Texas pollen station.
References
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