Eight-Hour Ozone High Value Days for 2026
as of June 19
The table below lists the daily maximum eight-hour ozone concentrations measured which meet or exceed 71 parts per billion in 2026 in each community where the TCEQ measures ozone. This information is updated hourly. Concentrations are color-highlighted based on the EPA-defined Air Quality Index colors. (See Interpreting the AQI.) Times shown are in 24-hour format and are Local Standard Time. Our convention is that the time reflects the beginning of the eight-hour average. All ozone measurements are in parts per billion.Use the selection boxes below to customize this report. You may select a different year, a different sorting order, or a different report format. Click on the Generate Report button once you have made your selections.
| 2026 Sorted by Maximum Value | |||||||
|---|---|---|---|---|---|---|---|
| Location | Region | CAMS | AQS | Date | Time | Value | POC |
| Texas City 34th St. C620 | 12 | 620 | 48_167_0056 | May 28 2026 | 11:00 | 122 | 1 N |
| Galveston 99th St. C1034/A320/X183 | 12 | 1034 | 48_167_1034 | May 28 2026 | 12:00 | 109 | 1 |
| CPS Pecan Valley C678 | 13 | 678 | 48_029_0055 | June 18 2026 | 11:00 | 104 | 1 N |
| Manvel Croix Park C84 | 12 | 84 | 48_039_1004 | May 13 2026 | 10:00 | 97 | 1 |
| Houston Croquet C409 | 12 | 409 | 48_201_0051 | May 13 2026 | 11:00 | 97 | 2 |
| Houston Bayland Park C53/A146 | 12 | 53 | 48_201_0055 | May 13 2026 | 11:00 | 95 | 1 |
| Houston Harvard Street C417 | 12 | 417 | 48_201_0417 | May 13 2026 | 11:00 | 95 | 1 |
| Tom Bass C558 | 12 | 558 | 48_201_0558 | May 13 2026 | 09:00 | 95 | 1 N |
| Hou.DeerPrk2 C35/235/1001/AFH139/FP239 | 12 | 35 | 48_201_1039 | May 28 2026 | 12:00 | 95 | 1 |
| Houston Monroe C406 | 12 | 406 | 48_201_0062 | May 13 2026 | 10:00 | 93 | 1 |
| Mercer Arboretum C557 | 12 | 557 | 48_201_0557 | May 29 2026 | 09:00 | 93 | 1 N |
| Park Place C416 | 12 | 416 | 48_201_0416 | May 13 2026 | 10:00 | 91 | 1 |
| Hou.DeerPrk2 C35/235/1001/AFH139/FP239 | 12 | 35 | 48_201_1039 | May 13 2026 | 11:00 | 89 | 1 |
| Denton Airport South C56/A163/X157 | 4 | 56 | 48_121_0034 | May 13 2026 | 11:00 | 88 | 1 |
| Texas City 34th St. C620 | 12 | 620 | 48_167_0056 | May 13 2026 | 11:00 | 88 | 1 N |
| Mercer Arboretum C557 | 12 | 557 | 48_201_0557 | May 14 2026 | 09:00 | 88 | 1 N |
| Bunker Hill Village C562 | 12 | 562 | 48_201_0562 | May 13 2026 | 10:00 | 88 | 1 N |
| SETRPC Mauriceville 42 C642/C311/C665 | 10 | 642 | 48_361_1100 | May 14 2026 | 10:00 | 88 | 1 N |
| Houston Aldine C8/AF108/X150 | 12 | 8 | 48_201_0024 | May 29 2026 | 10:00 | 87 | 2 |
| La Porte Sylvan Beach C556 | 12 | 556 | 48_201_0556 | May 28 2026 | 11:00 | 87 | 1 N |
| Conroe Relocated C78/A321 | 12 | 78 | 48_339_0078 | May 29 2026 | 12:00 | 87 | 1 |
| Frisco C31/C680 | 4 | 31 | 48_085_0005 | May 13 2026 | 11:00 | 86 | 1 |
| Pilot Point C1032 | 4 | 1032 | 48_121_1032 | May 13 2026 | 11:00 | 86 | 1 |
| Houston Aldine C8/AF108/X150 | 12 | 8 | 48_201_0024 | May 13 2026 | 11:00 | 86 | 2 |
| Lang C408 | 12 | 408 | 48_201_0047 | May 13 2026 | 11:00 | 86 | 2 |
| Houston Monroe C406 | 12 | 406 | 48_201_0062 | May 28 2026 | 12:00 | 86 | 1 |
| Huffman Wolf Road C563 | 12 | 563 | 48_201_0563 | May 14 2026 | 09:00 | 86 | 1 N |
| Conroe Relocated C78/A321 | 12 | 78 | 48_339_0078 | May 14 2026 | 11:00 | 86 | 1 |
| Frisco C31/C680 | 4 | 31 | 48_085_0005 | May 29 2026 | 11:00 | 84 | 1 |
| Galveston 99th St. C1034/A320/X183 | 12 | 1034 | 48_167_1034 | May 13 2026 | 11:00 | 84 | 1 |
| Houston East C1/G316 | 12 | 1 | 48_201_1034 | May 13 2026 | 11:00 | 84 | 2 |
| SETRPC Mauriceville 42 C642/C311/C665 | 10 | 642 | 48_361_1100 | May 4 2026 | 09:00 | 84 | 1 N |
| Pilot Point C1032 | 4 | 1032 | 48_121_1032 | May 14 2026 | 10:00 | 83 | 1 |
| Park Place C416 | 12 | 416 | 48_201_0416 | May 28 2026 | 12:00 | 83 | 1 |
| HRM-3 Haden Road C603/A114 | 12 | 603 | 48_201_0803 | May 28 2026 | 12:00 | 83 | 1 N |
| Seabrook Friendship Park C45 | 12 | 45 | 48_201_1050 | May 28 2026 | 12:00 | 83 | 1 |
| Killeen Skylark Field C1047 | 9 | 1047 | 48_027_1047 | May 14 2026 | 10:00 | 82 | 1 |
| West Houston C554 | 12 | 554 | 48_201_0554 | May 13 2026 | 10:00 | 82 | 1 N |
| Huffman Wolf Road C563 | 12 | 563 | 48_201_0563 | May 29 2026 | 09:00 | 82 | 1 N |
| HRM-3 Haden Road C603/A114 | 12 | 603 | 48_201_0803 | May 13 2026 | 11:00 | 82 | 1 N |
| SETRPC 40 Sabine Pass C640/C1654 | 10 | 640 | 48_245_0101 | May 28 2026 | 15:00 | 82 | 1 |
| Grapevine Fairway C70/A301/X182 | 4 | 70 | 48_439_3009 | May 13 2026 | 11:00 | 82 | 1 |
| Camp Bullis C58 | 13 | 58 | 48_029_0052 | May 13 2026 | 11:00 | 81 | 1 |
| Frisco C31/C680 | 4 | 31 | 48_085_0005 | May 14 2026 | 10:00 | 81 | 1 |
| Socorro Hueco C49/F312 | 6 | 49 | 48_141_0057 | June 13 2026 | 12:00 | 81 | 1 |
| La Porte Sylvan Beach C556 | 12 | 556 | 48_201_0556 | May 13 2026 | 10:00 | 81 | 1 N |
| Clinton C403/C304/AH113 | 12 | 403 | 48_201_1035 | May 28 2026 | 12:00 | 81 | 3 |
| SETRPC Mauriceville 42 C642/C311/C665 | 10 | 642 | 48_361_1100 | May 3 2026 | 10:00 | 81 | 1 N |
| CAPCOG Bastrop CAMS1612 | 11 | 1612 | 48_021_1612 | June 6 2026 | 22:00 | 80 | 1 N |
| CAPCOG Bastrop CAMS1612 | 11 | 1612 | 48_021_1612 | June 7 2026 | 00:00 | 80 | 1 N |
| Denton Airport South C56/A163/X157 | 4 | 56 | 48_121_0034 | May 14 2026 | 10:00 | 80 | 1 |
| Clinton C403/C304/AH113 | 12 | 403 | 48_201_1035 | May 13 2026 | 11:00 | 80 | 3 |
| Manvel Croix Park C84 | 12 | 84 | 48_039_1004 | May 28 2026 | 11:00 | 79 | 1 |
| Dallas North No.2 C63/C679 | 4 | 63 | 48_113_0075 | May 13 2026 | 11:00 | 79 | 1 |
| Denton Airport South C56/A163/X157 | 4 | 56 | 48_121_0034 | May 12 2026 | 12:00 | 79 | 1 |
| Houston Harvard Street C417 | 12 | 417 | 48_201_0417 | May 28 2026 | 11:00 | 79 | 1 |
| Mercer Arboretum C557 | 12 | 557 | 48_201_0557 | May 13 2026 | 10:00 | 79 | 1 N |
| Tom Bass C558 | 12 | 558 | 48_201_0558 | May 28 2026 | 11:00 | 79 | 1 N |
| Houston East C1/G316 | 12 | 1 | 48_201_1034 | May 28 2026 | 12:00 | 79 | 2 |
| Houston Aldine C8/AF108/X150 | 12 | 8 | 48_201_0024 | May 14 2026 | 10:00 | 78 | 2 |
| Houston Westhollow C410/C3003 | 12 | 410 | 48_201_0066 | May 13 2026 | 11:00 | 78 | 1 |
| Baytown Garth C1017 | 12 | 1017 | 48_201_1017 | May 14 2026 | 10:00 | 78 | 1 |
| Granbury C73/C681 | 4 | 73 | 48_221_0001 | May 14 2026 | 13:00 | 78 | 1 |
| SETRPC 40 Sabine Pass C640/C1654 | 10 | 640 | 48_245_0101 | May 14 2026 | 09:00 | 78 | 1 |
| Ft. Worth Northwest C13/AH302 | 4 | 13 | 48_439_1002 | May 13 2026 | 10:00 | 78 | 2 |
| Ft. Worth Northwest C13/AH302 | 4 | 13 | 48_439_1002 | May 14 2026 | 11:00 | 78 | 2 |
| CAPCOG East Austin C1619 | 11 | 1619 | 48_453_1619 | May 14 2026 | 10:00 | 78 | 1 N |
| Manvel Croix Park C84 | 12 | 84 | 48_039_1004 | May 12 2026 | 11:00 | 77 | 1 |
| Frisco C31/C680 | 4 | 31 | 48_085_0005 | May 12 2026 | 11:00 | 77 | 1 |
| Dallas Executive Airport C402 | 4 | 402 | 48_113_0087 | May 14 2026 | 12:00 | 77 | 1 |
| Pilot Point C1032 | 4 | 1032 | 48_121_1032 | May 29 2026 | 11:00 | 77 | 1 |
| Italy | 4 | 1044 | 48_139_1044 | May 14 2026 | 12:00 | 77 | 1 |
| Skyline Park C72 | 6 | 72 | 48_141_0058 | May 12 2026 | 11:00 | 77 | 1 |
| Northwest Harris Co. C26/A110/X154 | 12 | 26 | 48_201_0029 | May 3 2026 | 11:00 | 77 | 2 |
| Dripping Springs School C614 | 11 | 614 | 48_209_0614 | May 13 2026 | 12:00 | 77 | 1 N |
| Dripping Springs School C614 | 11 | 614 | 48_209_0614 | May 14 2026 | 10:00 | 77 | 1 N |
| SETRPC 40 Sabine Pass C640/C1654 | 10 | 640 | 48_245_0101 | May 13 2026 | 11:00 | 77 | 1 |
| CAPCOG East Austin C1619 | 11 | 1619 | 48_453_1619 | May 13 2026 | 10:00 | 77 | 1 N |
| CAPCOG East Austin C1619 | 11 | 1619 | 48_453_1619 | May 28 2026 | 11:00 | 77 | 1 N |
| Temple Georgia C1045 | 9 | 1045 | 48_027_1045 | May 14 2026 | 11:00 | 76 | 1 |
| San Antonio Northwest C23 | 13 | 23 | 48_029_0032 | May 13 2026 | 12:00 | 76 | 2 |
| Pilot Point C1032 | 4 | 1032 | 48_121_1032 | May 12 2026 | 11:00 | 76 | 1 |
| Ascarate Park SE C37/A332/A172/X159 | 6 | 37 | 48_141_0055 | May 12 2026 | 11:00 | 76 | 1 |
| Seabrook Friendship Park C45 | 12 | 45 | 48_201_1050 | May 13 2026 | 11:00 | 76 | 1 |
| West Orange C9/A141 | 10 | 9 | 48_361_1001 | May 14 2026 | 11:00 | 76 | 2 |
| SETRPC Mauriceville 42 C642/C311/C665 | 10 | 642 | 48_361_1100 | May 29 2026 | 08:00 | 76 | 1 N |
| Keller C17 | 4 | 17 | 48_439_2003 | May 13 2026 | 11:00 | 76 | 2 |
| Grapevine Fairway C70/A301/X182 | 4 | 70 | 48_439_3009 | May 14 2026 | 10:00 | 76 | 1 |
| Houston Croquet C409 | 12 | 409 | 48_201_0051 | May 12 2026 | 11:00 | 75 | 2 |
| Crosby Library C553 | 12 | 553 | 48_201_0553 | May 29 2026 | 09:00 | 75 | 1 N |
| Meyer Park C561 | 12 | 561 | 48_201_0561 | May 13 2026 | 11:00 | 75 | 1 N |
| Lynchburg Ferry C1015/A165 | 12 | 1015 | 48_201_1015 | May 28 2026 | 12:00 | 75 | 1 |
| Dripping Springs School C614 | 11 | 614 | 48_209_0614 | May 12 2026 | 12:00 | 75 | 1 N |
| Kaufman C71/A304/X071 | 4 | 71 | 48_257_0005 | May 14 2026 | 11:00 | 75 | 1 |
| Eagle Mountain Lake C75 | 4 | 75 | 48_439_0075 | May 14 2026 | 10:00 | 75 | 1 |
| Arlington Municipal Airport C61 | 4 | 61 | 48_439_3011 | May 14 2026 | 12:00 | 75 | 1 |
| Victoria C87 | 14 | 87 | 48_469_0003 | May 13 2026 | 10:00 | 75 | 1 |
| Lockhart C1604 | 11 | 1604 | 48_055_1604 | May 14 2026 | 10:00 | 74 | 1 N |
| Frisco C31/C680 | 4 | 31 | 48_085_0005 | May 22 2026 | 12:00 | 74 | 1 |
| Dallas North No.2 C63/C679 | 4 | 63 | 48_113_0075 | May 14 2026 | 12:00 | 74 | 1 |
| Dallas North No.2 C63/C679 | 4 | 63 | 48_113_0075 | May 28 2026 | 12:00 | 74 | 1 |
| Channelview C15/AH115 | 12 | 15 | 48_201_0026 | May 28 2026 | 13:00 | 74 | 3 |
| Houston Bayland Park C53/A146 | 12 | 53 | 48_201_0055 | May 12 2026 | 11:00 | 74 | 1 |
| Houston Harvard Street C417 | 12 | 417 | 48_201_0417 | May 29 2026 | 10:00 | 74 | 1 |
| Crosby Library C553 | 12 | 553 | 48_201_0553 | May 14 2026 | 09:00 | 74 | 1 N |
| Meyer Park C561 | 12 | 561 | 48_201_0561 | May 14 2026 | 09:00 | 74 | 1 N |
| Wallisville Road C617 | 12 | 617 | 48_201_0617 | May 14 2026 | 09:00 | 74 | 1 N |
| Lynchburg Ferry C1015/A165 | 12 | 1015 | 48_201_1015 | May 13 2026 | 11:00 | 74 | 1 |
| Baytown Garth C1017 | 12 | 1017 | 48_201_1017 | May 29 2026 | 09:00 | 74 | 1 |
| Dripping Springs School C614 | 11 | 614 | 48_209_0614 | May 3 2026 | 11:00 | 74 | 1 N |
| Parker County C76 | 4 | 76 | 48_367_0081 | May 14 2026 | 14:00 | 74 | 1 |
| Eagle Mountain Lake C75 | 4 | 75 | 48_439_0075 | May 13 2026 | 11:00 | 74 | 1 |
| Austin North Hills Drive C3/A322 | 11 | 3 | 48_453_0014 | May 13 2026 | 11:00 | 74 | 2 |
| Camp Bullis C58 | 13 | 58 | 48_029_0052 | May 28 2026 | 11:00 | 73 | 1 |
| Houston Aldine C8/AF108/X150 | 12 | 8 | 48_201_0024 | May 28 2026 | 11:00 | 73 | 2 |
| Northwest Harris Co. C26/A110/X154 | 12 | 26 | 48_201_0029 | May 13 2026 | 10:00 | 73 | 2 |
| Houston Harvard Street C417 | 12 | 417 | 48_201_0417 | May 14 2026 | 09:00 | 73 | 1 |
| HRM-3 Haden Road C603/A114 | 12 | 603 | 48_201_0803 | May 14 2026 | 09:00 | 73 | 1 N |
| Hamshire C64/C654 | 10 | 64 | 48_245_0022 | May 14 2026 | 09:00 | 73 | 1 |
| Waco Mazanec C1037 | 9 | 1037 | 48_309_1037 | May 14 2026 | 10:00 | 73 | 1 |
| Ft. Worth Northwest C13/AH302 | 4 | 13 | 48_439_1002 | June 16 2026 | 09:00 | 73 | 2 |
| Keller C17 | 4 | 17 | 48_439_2003 | May 14 2026 | 11:00 | 73 | 2 |
| Grapevine Fairway C70/A301/X182 | 4 | 70 | 48_439_3009 | May 12 2026 | 12:00 | 73 | 1 |
| Audubon C38 | 11 | 38 | 48_453_0020 | May 13 2026 | 11:00 | 73 | 1 |
| Audubon C38 | 11 | 38 | 48_453_0020 | May 14 2026 | 10:00 | 73 | 1 |
| CAPCOG Lake Georgetown C690 | 11 | 690 | 48_491_0690 | May 14 2026 | 10:00 | 73 | 1 N |
| Dallas Executive Airport C402 | 4 | 402 | 48_113_0087 | May 13 2026 | 10:00 | 72 | 1 |
| Denton Airport South C56/A163/X157 | 4 | 56 | 48_121_0034 | May 22 2026 | 12:00 | 72 | 1 |
| Northwest Harris Co. C26/A110/X154 | 12 | 26 | 48_201_0029 | May 14 2026 | 10:00 | 72 | 2 |
| Baytown Wetlands Center C552 | 12 | 552 | 48_201_0552 | May 13 2026 | 10:00 | 72 | 1 N |
| Baytown Wetlands Center C552 | 12 | 552 | 48_201_0552 | May 28 2026 | 12:00 | 72 | 1 N |
| Meyer Park C561 | 12 | 561 | 48_201_0561 | May 3 2026 | 10:00 | 72 | 1 N |
| Baytown Garth C1017 | 12 | 1017 | 48_201_1017 | May 28 2026 | 13:00 | 72 | 1 |
| Greenville C1006/A198 | 4 | 1006 | 48_231_1006 | May 14 2026 | 11:00 | 72 | 1 |
| Cleburne Airport C77/C682 | 4 | 77 | 48_251_0003 | May 14 2026 | 13:00 | 72 | 1 |
| Ft. Worth Northwest C13/AH302 | 4 | 13 | 48_439_1002 | May 23 2026 | 11:00 | 72 | 2 |
| Arlington Municipal Airport C61 | 4 | 61 | 48_439_3011 | May 9 2026 | 12:00 | 72 | 1 |
| Austin North Hills Drive C3/A322 | 11 | 3 | 48_453_0014 | May 14 2026 | 10:00 | 72 | 2 |
| St. Edwards University C1605 | 11 | 1605 | 48_453_1605 | May 13 2026 | 11:00 | 72 | 1 N |
| Temple Georgia C1045 | 9 | 1045 | 48_027_1045 | May 13 2026 | 10:00 | 71 | 1 |
| Elm Creek Elementary C501 | 13 | 501 | 48_029_0501 | May 12 2026 | 11:00 | 71 | 1 N |
| Fair Oaks Ranch C502 | 13 | 502 | 48_029_0502 | May 13 2026 | 11:00 | 71 | 1 N |
| Fair Oaks Ranch C502 | 13 | 502 | 48_029_0502 | May 28 2026 | 11:00 | 71 | 1 N |
| Manvel Croix Park C84 | 12 | 84 | 48_039_1004 | May 25 2026 | 10:00 | 71 | 1 |
| Lockhart C1604 | 11 | 1604 | 48_055_1604 | May 3 2026 | 11:00 | 71 | 1 N |
| Lockhart C1604 | 11 | 1604 | 48_055_1604 | May 13 2026 | 11:00 | 71 | 1 N |
| Bulverde Elementary C503 | 13 | 503 | 48_091_0503 | May 13 2026 | 11:00 | 71 | 1 N |
| AACOG City of Garden Ridge C505 | 13 | 505 | 48_091_0505 | May 13 2026 | 11:00 | 71 | 1 N |
| Dallas North No.2 C63/C679 | 4 | 63 | 48_113_0075 | May 12 2026 | 11:00 | 71 | 1 |
| Dallas North No.2 C63/C679 | 4 | 63 | 48_113_0075 | May 29 2026 | 10:00 | 71 | 1 |
| Dallas Executive Airport C402 | 4 | 402 | 48_113_0087 | May 9 2026 | 11:00 | 71 | 1 |
| Ivanhoe C414/F514 | 6 | 414 | 48_141_0029 | June 13 2026 | 12:00 | 71 | 1 |
| Longview C19/A127/C644 | 5 | 19 | 48_183_0001 | May 13 2026 | 10:00 | 71 | 2 |
| Houston Croquet C409 | 12 | 409 | 48_201_0051 | May 24 2026 | 10:00 | 71 | 2 |
| Wallisville Road C617 | 12 | 617 | 48_201_0617 | May 29 2026 | 09:00 | 71 | 1 N |
| Corpus Christi Tuloso C21 | 14 | 21 | 48_355_0026 | May 13 2026 | 10:00 | 71 | 1 |
| Parker County C76 | 4 | 76 | 48_367_0081 | June 3 2026 | 11:00 | 71 | 1 |
| Rockwall Heath C69 | 4 | 69 | 48_397_0001 | May 14 2026 | 11:00 | 71 | 1 |
| Eagle Mountain Lake C75 | 4 | 75 | 48_439_0075 | May 23 2026 | 13:00 | 71 | 1 |
| Ft. Worth Northwest C13/AH302 | 4 | 13 | 48_439_1002 | May 9 2026 | 11:00 | 71 | 2 |
| Grapevine Fairway C70/A301/X182 | 4 | 70 | 48_439_3009 | May 29 2026 | 10:00 | 71 | 1 |
| CAPCOG Taylor Murphy Park C1629 | 11 | 1629 | 48_491_1629 | May 14 2026 | 10:00 | 71 | 1 N |
| POC (Parameter Occurrence Code): a code used to correctly separate data from multiple instruments at one site. | |||||||
| N - Data from this instrument does not meet EPA quality assurance criteria and cannot be used for regulatory purposes. | |||||||
Interpreting the AQI
Each NAAQS pollutant has a separate AQI scale, with an AQI rating of 100 corresponding to the concentration of the Federal Standard for that pollutant. Additional information about the AQI and how it can be used is available from the EPA's AirNow web site.
Place your mouse pointer over the scale displayed above to view information about the Air Quality Index, and each of the rating levels.
The actual index calculation is different for each parameter measured and is specified by the EPA. The following table shows the various breakpoints used in calculating the AQI.
| AQI Breakpoint Definitions | ||
|---|---|---|
| AQI Range | 1hr Ozone in ppm |
8hr Ozone in ppm |
| 0 - 50 | Not Defined | 0 - 0.054 |
| 51 - 100 | Not Defined | 0.055 - 0.070 |
| 101 - 150 | 0.125 - 0.164 | 0.071 - 0.085 |
| 151 - 200 | 0.165 - 0.204 | 0.086 - 0.105 |
| 201 - 300 | 0.205 - 0.404 | 0.106 - 0.200 |
| 301 - 400 | 0.405 - 0.504 | Not Defined |
| 401 - 500 | Not Defined | Not Defined |
| 500+ | Not Defined | Not Defined |
- The AQI for ozone is based on either the peak eight-hour running average since midnight OR the peak one-hour measurement since midnight.
PLEASE NOTE: This data has not been verified by the TCEQ and may change. This is the most current data, but it is not official until it has been certified by our technical staff. Data is collected from TCEQ ambient monitoring sites and may include data collected by other outside agencies. This data is updated hourly. All times shown are in local standard time unless otherwise indicated.
Following EPA reporting guidelines, negative values may be displayed in our hourly criteria air quality data, down to the negative of the EPA listed Method Detection Limit (MDL) for the particular instrument that made the measurements. The reported concentrations can be negative due to zero drift in the electronic instrument output, data logger channel, or calibration adjustments to the data. Prior to 1/1/2013, slightly negative values were automatically set to zero.